diff --git a/src/identity/configuration/IdentityProviderFactory.ts b/src/identity/configuration/IdentityProviderFactory.ts
index 10bf782ad..ea442ab21 100644
--- a/src/identity/configuration/IdentityProviderFactory.ts
+++ b/src/identity/configuration/IdentityProviderFactory.ts
@@ -19,6 +19,7 @@ import type { ResponseWriter } from '../../http/output/ResponseWriter';
 import { BasicRepresentation } from '../../http/representation/BasicRepresentation';
 import { getLoggerFor } from '../../logging/LogUtil';
 import type { KeyValueStorage } from '../../storage/keyvalue/KeyValueStorage';
+import { BadRequestHttpError } from '../../util/errors/BadRequestHttpError';
 import { InternalServerError } from '../../util/errors/InternalServerError';
 import { RedirectHttpError } from '../../util/errors/RedirectHttpError';
 import { guardStream } from '../../util/GuardedStream';
@@ -398,6 +399,22 @@ export class IdentityProviderFactory implements ProviderFactory {
         }
       }
 
+      // A client not being found is quite often the result of cookies being stored by the authn client,
+      // so we want to provide a more detailed error message explaining what to do.
+      if (oidcError.error_description === 'client is invalid' && oidcError.error_detail === 'client not found') {
+        const unknownClientError = new BadRequestHttpError(
+          'Unknown client, you might need to clear the local storage on the client.', {
+            errorCode: 'E0003',
+            details: {
+              client_id: ctx.request.query.client_id,
+              redirect_uri: ctx.request.query.redirect_uri,
+            },
+          },
+        );
+        unknownClientError.stack = error.stack;
+        error = unknownClientError;
+      }
+
       const result = await this.errorHandler.handleSafe({ error, request: guardStream(ctx.req) });
       await this.responseWriter.handleSafe({ response: ctx.res, result });
     };
diff --git a/templates/error/descriptions/E0003.md.hbs b/templates/error/descriptions/E0003.md.hbs
new file mode 100644
index 000000000..1bd8513e3
--- /dev/null
+++ b/templates/error/descriptions/E0003.md.hbs
@@ -0,0 +1,26 @@
+# Authenticating with unknown client
+You are trying to log in to an application,
+but we can't proceed
+because the app is using invalid settings.
+
+To force the app to send us the right details,
+delete the local storage in your browser for the site that sent you here.
+Based on the data the app sent us,
+this is probably `{{ redirect_uri }}`.
+
+## Detailed error information
+We received a request from a client with ID `{{ client_id }}`,
+but this client is not registered with the server.
+
+Probably,
+this client was registered with the server in the past,
+but it is no longer recognized
+because some internal server data was removed.
+Your data is still safe;
+we just don't recognize the app's previous authentication anymore.
+
+Because your browser still has the old authentication settings stored,
+it tries to use them instead of setting up new ones.
+By clearing those settings,
+the application should automatically create a new client,
+allowing you to log in again.
diff --git a/test/unit/identity/configuration/IdentityProviderFactory.test.ts b/test/unit/identity/configuration/IdentityProviderFactory.test.ts
index 05701709d..7adf696e7 100644
--- a/test/unit/identity/configuration/IdentityProviderFactory.test.ts
+++ b/test/unit/identity/configuration/IdentityProviderFactory.test.ts
@@ -59,6 +59,10 @@ describe('An IdentityProviderFactory', (): void => {
       res: {},
       request: {
         href: 'http://example.com/idp/',
+        query: {
+          client_id: 'CLIENT_ID',
+          redirect_uri: 'REDIRECT_URI',
+        },
       },
       accepts: jest.fn().mockReturnValue('type'),
     } as any;
@@ -235,6 +239,32 @@ describe('An IdentityProviderFactory', (): void => {
     expect(error.stack).toContain('Error: bad data - more info - more details');
   });
 
+  it('throws a specific error for unknown clients.', async(): Promise<void> => {
+    const provider = await factory.getProvider() as any;
+    const { config } = provider as { config: Configuration };
+
+    const error = new Error('invalid_client') as errors.OIDCProviderError;
+    error.error_description = 'client is invalid';
+    error.error_detail = 'client not found';
+
+    await expect((config.renderError as any)(ctx, {}, error)).resolves.toBeUndefined();
+    expect(errorHandler.handleSafe).toHaveBeenCalledTimes(1);
+    expect(errorHandler.handleSafe)
+      .toHaveBeenLastCalledWith({ error: expect.objectContaining({
+        statusCode: 400,
+        name: 'BadRequestHttpError',
+        message: 'Unknown client, you might need to clear the local storage on the client.',
+        errorCode: 'E0003',
+        details: {
+          client_id: 'CLIENT_ID',
+          redirect_uri: 'REDIRECT_URI',
+        },
+      }),
+      request: ctx.req });
+    expect(responseWriter.handleSafe).toHaveBeenCalledTimes(1);
+    expect(responseWriter.handleSafe).toHaveBeenLastCalledWith({ response: ctx.res, result: { statusCode: 500 }});
+  });
+
   it('adds middleware to make the OIDC provider think the request wants HTML.', async(): Promise<void> => {
     const provider = await factory.getProvider() as any;
     const use = provider.use as jest.Mock<ReturnType<Koa['use']>, Parameters<Koa['use']>>;