diff --git a/src/core/features/login/pages/credentials/credentials.ts b/src/core/features/login/pages/credentials/credentials.ts
index b8638cab5..9ee00529a 100644
--- a/src/core/features/login/pages/credentials/credentials.ts
+++ b/src/core/features/login/pages/credentials/credentials.ts
@@ -181,7 +181,7 @@ export class CoreLoginCredentialsPage implements OnInit, OnDestroy {
 
         try {
             if (!this.siteCheck) {
-                this.siteCheck = await CoreSites.checkSite(this.site.siteUrl, protocol);
+                this.siteCheck = await CoreSites.checkSite(this.site.siteUrl, protocol, 'Credentials page');
                 this.siteCheck.config && this.site.setPublicConfig(this.siteCheck.config);
             }
 
diff --git a/src/core/features/login/pages/site/site.ts b/src/core/features/login/pages/site/site.ts
index 02df4ee1d..a454bd12e 100644
--- a/src/core/features/login/pages/site/site.ts
+++ b/src/core/features/login/pages/site/site.ts
@@ -333,14 +333,14 @@ export class CoreLoginSitePage implements OnInit {
             let checkResult: CoreSiteCheckResponse;
 
             try {
-                checkResult = await CoreSites.checkSite(url);
+                checkResult = await CoreSites.checkSite(url, undefined, 'Site URL page');
             } catch (error) {
                 // Attempt guessing the domain if the initial check failed
                 const domain = CoreUrl.guessMoodleDomain(url);
 
                 if (domain && domain != url) {
                     try {
-                        checkResult = await CoreSites.checkSite(domain);
+                        checkResult = await CoreSites.checkSite(domain, undefined, 'Site URL page');
                     } catch (secondError) {
                         // Try to use the first error.
                         modal.dismiss();
@@ -608,7 +608,7 @@ export class CoreLoginSitePage implements OnInit {
 
         try {
             // Check if site uses SSO.
-            const siteCheck = await CoreSites.checkSite(siteUrl);
+            const siteCheck = await CoreSites.checkSite(siteUrl, undefined, 'Site URL page');
 
             await CoreSites.checkApplication(siteCheck.config);
 
diff --git a/src/core/services/sites.ts b/src/core/services/sites.ts
index 0a6d676d5..1dfdd725e 100644
--- a/src/core/services/sites.ts
+++ b/src/core/services/sites.ts
@@ -286,14 +286,18 @@ export class CoreSitesProvider {
      *
      * @param siteUrl URL of the site to check.
      * @param protocol Protocol to use first.
+     * @param origin Origin of this check site call.
      * @returns A promise resolved when the site is checked.
      */
-    async checkSite(siteUrl: string, protocol: string = 'https://'): Promise<CoreSiteCheckResponse> {
+    async checkSite(siteUrl: string, protocol: string = 'https://', origin = 'unknown'): Promise<CoreSiteCheckResponse> {
         // The formatURL function adds the protocol if is missing.
         siteUrl = CoreUrl.formatURL(siteUrl);
 
         if (!CoreUrl.isHttpURL(siteUrl)) {
-            throw new CoreError(Translate.instant('core.login.invalidsite'));
+            throw new CoreError(Translate.instant('core.login.invalidsite'), {
+                code: 'invalidprotocol',
+                details: `URL contains an invalid protocol when checking site.<br><br>Origin: ${origin}.<br><br>URL: ${siteUrl}.`,
+            });
         }
 
         if (!CoreNetwork.isOnline()) {
diff --git a/src/core/services/urlschemes.ts b/src/core/services/urlschemes.ts
index f7131a788..65cd87bc4 100644
--- a/src/core/services/urlschemes.ts
+++ b/src/core/services/urlschemes.ts
@@ -44,6 +44,22 @@ export class CoreCustomURLSchemesProvider {
         this.logger = CoreLogger.getInstance('CoreCustomURLSchemesProvider');
     }
 
+    /**
+     * Create a CoreCustomURLSchemesHandleError to be used when treating a URL that doesn't have a valid scheme.
+     *
+     * @param url URL that caused the error.
+     * @param data Data obtained from the URL (if any).
+     * @returns Error.
+     */
+    protected createInvalidSchemeError(url: string, data?: CoreCustomURLSchemesParams): CoreCustomURLSchemesHandleError {
+        const defaultError = new CoreError(Translate.instant('core.login.invalidsite'), {
+            code: 'invalidurlscheme',
+            details: `Error when treating a URL scheme, it seems the URL is not valid.<br><br>URL: ${url}`,
+        });
+
+        return new CoreCustomURLSchemesHandleError(defaultError, data);
+    }
+
     /**
      * Given some data of a custom URL with a token, create a site if it needs to be created.
      *
@@ -62,7 +78,7 @@ export class CoreCustomURLSchemesProvider {
 
             if (!data.siteUrl.match(/^https?:\/\//)) {
                 // URL doesn't have a protocol and it's required to be able to create the site. Check which one to use.
-                const result = await CoreSites.checkSite(data.siteUrl);
+                const result = await CoreSites.checkSite(data.siteUrl, undefined, 'URL scheme create site');
 
                 data.siteUrl = result.siteUrl;
 
@@ -90,7 +106,13 @@ export class CoreCustomURLSchemesProvider {
      */
     async handleCustomURL(url: string): Promise<void> {
         if (!this.isCustomURL(url)) {
-            throw new CoreCustomURLSchemesHandleError(null);
+            throw this.createInvalidSchemeError(url);
+        }
+
+        // Check if there is nothing valid after the URL scheme.
+        const urlWithoutScheme = this.removeCustomURLScheme(url).trim();
+        if (!urlWithoutScheme || urlWithoutScheme.match(/^\/?(#.*)?\/?$/)) {
+            throw this.createInvalidSchemeError(url);
         }
 
         /* First check that this URL hasn't been treated a few seconds ago. The function that handles custom URL schemes already
@@ -202,7 +224,7 @@ export class CoreCustomURLSchemesProvider {
 
             } else {
                 // Site not stored. Try to add the site.
-                const result = await CoreSites.checkSite(data.siteUrl);
+                const result = await CoreSites.checkSite(data.siteUrl, undefined, `URL scheme redirect: ${url}`);
 
                 // Site exists. We'll allow to add it.
                 modal.dismiss(); // Dismiss modal so it doesn't collide with confirms.
@@ -211,7 +233,12 @@ export class CoreCustomURLSchemesProvider {
             }
 
         } catch (error) {
-            throw new CoreCustomURLSchemesHandleError(error, data);
+            if (!error || !CoreErrorHelper.getErrorMessageFromError(error)) {
+                // Use a default error.
+                this.createInvalidSchemeError(url, data);
+            } else {
+                throw new CoreCustomURLSchemesHandleError(error, data);
+            }
         } finally {
             modal.dismiss();
 
@@ -230,7 +257,7 @@ export class CoreCustomURLSchemesProvider {
      */
     protected async getCustomURLData(url: string): Promise<CoreCustomURLSchemesParams> {
         if (!this.isCustomURL(url)) {
-            throw new CoreCustomURLSchemesHandleError(null);
+            throw this.createInvalidSchemeError(url);
         }
 
         // App opened using custom URL scheme.
@@ -283,7 +310,7 @@ export class CoreCustomURLSchemesProvider {
      */
     protected async getCustomURLLinkData(url: string): Promise<CoreCustomURLSchemesParams> {
         if (!this.isCustomURLLink(url)) {
-            throw new CoreCustomURLSchemesHandleError(null);
+            throw this.createInvalidSchemeError(url);
         }
 
         // App opened using custom URL scheme.
@@ -345,7 +372,7 @@ export class CoreCustomURLSchemesProvider {
      */
     protected async getCustomURLTokenData(url: string): Promise<CoreCustomURLSchemesParams> {
         if (!this.isCustomURLToken(url)) {
-            throw new CoreCustomURLSchemesHandleError(null);
+            throw this.createInvalidSchemeError(url);
         }
 
         if (CoreSSO.isSSOAuthenticationOngoing()) {
@@ -358,6 +385,7 @@ export class CoreCustomURLSchemesProvider {
         this.logger.debug('App launched by URL with an SSO');
 
         // Delete the sso scheme from the URL.
+        const originalUrl = url;
         url = this.removeCustomURLTokenScheme(url);
 
         // Some platforms like Windows add a slash at the end. Remove it.
@@ -371,7 +399,11 @@ export class CoreCustomURLSchemesProvider {
             // Error decoding the parameter.
             this.logger.error('Error decoding parameter received for login SSO');
 
-            throw new CoreCustomURLSchemesHandleError(null);
+            throw new CoreCustomURLSchemesHandleError(new CoreError(Translate.instant('core.login.invalidsite'), {
+                code: 'errordecodingparameter',
+                details: `Error when trying to decode base 64 string.<br><br>URL: ${originalUrl}<br><br>Text to decode: ${url}` +
+                    `<br><br>Error: ${CoreErrorHelper.getErrorMessageFromError(err)}`,
+            }));
         }
 
         const data: CoreCustomURLSchemesParams = await CoreLoginHelper.validateBrowserSSOLogin(url);
@@ -491,14 +523,17 @@ export class CoreCustomURLSchemesProvider {
      * @param error Error data.
      */
     treatHandleCustomURLError(error: CoreCustomURLSchemesHandleError): void {
-        if (error.error == 'Duplicated') {
+        if (error.error === 'Duplicated') {
             // Duplicated request
         } else if (CoreWSError.isWebServiceError(error.error) && error.data && error.data.isSSOToken) {
             // An error occurred, display the error and logout the user.
             CoreLoginHelper.treatUserTokenError(error.data.siteUrl, <CoreWSError> error.error);
             CoreSites.logout();
         } else {
-            CoreDomUtils.showErrorModalDefault(error.error, Translate.instant('core.login.invalidsite'));
+            CoreDomUtils.showErrorModal(error.error ?? new CoreError(Translate.instant('core.login.invalidsite'), {
+                code: 'unknownerror',
+                details: 'Unknown error when treating a URL scheme.',
+            }));
         }
     }