Final fix.

backend/src/Squidex/Areas/IdentityServer/Config/IdentityServerServices.cs

@@ -10,8 +10,10 @@ using Microsoft.AspNetCore.DataProtection;
 using Microsoft.AspNetCore.DataProtection.KeyManagement;
 using Microsoft.AspNetCore.DataProtection.Repositories;
 using Microsoft.AspNetCore.Identity;
+using Microsoft.Extensions.Options;
 using OpenIddict.Abstractions;
 using OpenIddict.Server;
+using Squidex.Config;
 using Squidex.Domain.Users;
 using Squidex.Domain.Users.InMemory;
 using Squidex.Hosting;
@@ -84,16 +86,6 @@ namespace Squidex.Areas.IdentityServer.Config
                             .SetOrder(AttachTokenParameters.Descriptor.Order + 1);
                     });
 
-                    var identityServer = Constants.PrefixIdentityServer;
-
-                    builder.SetAuthorizationEndpointUris($"{identityServer}/connect/authorize");
-                    builder.SetConfigurationEndpointUris($"{identityServer}/.well-known/openid-configuration");
-                    builder.SetCryptographyEndpointUris($"{identityServer}/.well-known/jwks");
-                    builder.SetIntrospectionEndpointUris($"{identityServer}/connect/introspect");
-                    builder.SetLogoutEndpointUris($"{identityServer}/connect/logout");
-                    builder.SetTokenEndpointUris($"{identityServer}/connect/token");
-                    builder.SetUserinfoEndpointUris($"{identityServer}/connect/userinfo");
-
                     builder.SetAccessTokenLifetime(TimeSpan.FromDays(30));
 
                     builder.DisableAccessTokenEncryption();
@@ -127,8 +119,51 @@ namespace Squidex.Areas.IdentityServer.Config
             {
                 var urlGenerator = services.GetRequiredService<IUrlGenerator>();
 
-                options.Issuer = new Uri(urlGenerator.BuildUrl());
+                var identityPrefix = Constants.PrefixIdentityServer;
+                var identityOptions = services.GetRequiredService<IOptions<MyIdentityOptions>>().Value;
+
+                Func<string, Uri> buildUrl;
+
+                if (identityOptions.MultipleDomains)
+                {
+                    buildUrl = url => new Uri($"{identityPrefix}{url}", UriKind.Relative);
+
+                    options.Issuer = new Uri(urlGenerator.BuildUrl());
+                }
+                else
+                {
+                    buildUrl = url => new Uri(urlGenerator.BuildUrl($"{identityPrefix}{url}", false));
+
+                    options.Issuer = new Uri(urlGenerator.BuildUrl(identityPrefix, false));
+                }
+
+                options.AuthorizationEndpointUris.SetEndpoint(
+                    buildUrl("/connect/authorize"));
+
+                options.IntrospectionEndpointUris.SetEndpoint(
+                    buildUrl("/connect/introspect"));
+
+                options.LogoutEndpointUris.SetEndpoint(
+                    buildUrl("/connect/logout"));
+
+                options.TokenEndpointUris.SetEndpoint(
+                    buildUrl("/connect/token"));
+
+                options.UserinfoEndpointUris.SetEndpoint(
+                    buildUrl("/connect/userinfo"));
+
+                options.CryptographyEndpointUris.SetEndpoint(
+                    buildUrl("/.well-known/jwks"));
+
+                options.ConfigurationEndpointUris.SetEndpoint(
+                    buildUrl("/.well-known/openid-configuration"));
             });
         }
+
+        private static void SetEndpoint(this List<Uri> endpointUris, Uri uri)
+        {
+            endpointUris.Clear();
+            endpointUris.Add(uri);
+        }
     }
 }

backend/src/Squidex/Config/MyIdentityOptions.cs

@@ -37,6 +37,8 @@ namespace Squidex.Config
 
         public string MicrosoftTenant { get; set; }
 
+        public Dictionary<string, string[]> OidcRoleMapping { get; set; }
+
         public string OidcName { get; set; }
 
         public string OidcClient { get; set; }
@@ -57,14 +59,14 @@ namespace Squidex.Config
 
         public bool OidcGetClaimsFromUserInfoEndpoint { get; set; }
 
-        public Dictionary<string, string[]> OidcRoleMapping { get; set; }
-
         public bool AdminRecreate { get; set; }
 
         public bool AllowPasswordAuth { get; set; }
 
         public bool LockAutomatically { get; set; }
 
+        public bool MultipleDomains { get; set; }
+
         public bool NoConsent { get; set; }
 
         public bool RequiresHttps { get; set; }

backend/src/Squidex/appsettings.json

@@ -513,6 +513,9 @@
         "microsoftSecret": "idWbANxNYEF4cB368WXJhjN",
         "microsoftTenant": null,
 
+        // Set this to true if you use multiple domains.
+        "multipleDomains": false,
+
         // Settings for your custom oidc server.
         "oidcName": "OIDC",
         "oidcAuthority": "",