Wrong redirect if already authenticated with SSO

Dear devs,

thank you very much for this app. I integrated Nextcloud with keycloak and see a redirect error if I am already authenticated with Keycloak.

Steps to reproduce:

1. Get a valid session with keycloak by logging in to some other service on the same realm
2. Access the nextcloud base url (https://xxx.tld/nextcloud/)
3. Observe the following 302 redirects: (https://xxx.tld/nextcloud/login --> https://xxx.tld/nextcloud/apps/user_oidc/login/1 --> https://xxx.tld/nextcloud/index.php_oidc/login/1)
4. The last redirect results in a 404 not found, because the URL does not exists. Note the index.php_oidc/login/1 part that looks wrong to me.

What I think is happening

user_oidc is checking with keycloak for authentication, realizes the session is already authenticated with keycloak, authenticates the user with nextcloud and immediately tries to redirect back to nextcloud. But somehow the redirect URL gets mangled.
This error does not occur when logging in directly with nextlcoud, e.g. with no prior authentication with keycloak.

Cheers
Robert

Reproduce the redirect sequence from /login through apps/user_oidc/login/1, focusing on the transition to the malformed index.php_oidc/login/1 URL. Trace the already-authenticated SSO redirect handling and verify that the flow returns to a valid Nextcloud URL without a 404.