Ah, ok. So there isn’t a webmail there?
By the way, is it correct that when I configure LDAP only the account I log in with will be created? Isn’t there a way to get Stalwart to read my accounts?
AFAIK Stalwart is a server core. It does not ship client calendar or webmail. You can use any client webmail (Thunderbird, Outlook, Gmail, Roundcube…) to access your messages.
Concerning LDAP I cannot answer your question.
Thanks for your help!
Which OIDC are you using? Could you share your setup/config?
I’m not - hence my confusion. Things seem to work now. 
I asked @DorianCoding
I don’t but web application does.
If you want a webmailer, have a look at bulwark, which ist built for stalwart: GitHub - bulwarkmail/webmail: JMAP Webmail built for the 21st Century. A modern, self-hosted email client for Stalwart Mail Server powered by the JMAP protocol. Email, calendar, contacts and files. Fast, private, and open source. · GitHub
Exactly the same issue here. Please let me know if you have a solution, I’ll let you know if I get anywhere.
I think that this is most because the endpoint is used when Stalwart acts like an idp for other apps. It’s not really a sign for malfunction.
The auth is actually succeeding (your logs show auth.success and a 200), so this isn’t a credential or certificate problem. What you’re seeing is that the bare /login page is the OIDC authorization endpoint: opened directly it has no redirect_uri to send you back to, so it renders “Temporary server failure”. The port 443/80/8080 and proxy detours earlier in the thread were red herrings.
Log in at /admin (admin console) or /account (user portal) and it injects the OAuth parameters for you. @root_root and @DorianCoding had this right above.