HTTP 503 Error during frontend login

Summary

Internal Server Error (HTTP 503) after login via Keycloak (my.typo3.org).

Steps to reproduce

(How one can reproduce the issue - this is very important)

Example URL

https://typo3.org/oauth2/callback?oauth2-provider=keycloak&logintype=login&state=[redacted] (not directly applicable, use the login mechanism via my.typo3.org instead)

What is the current bug behavior?

HTTP 503 Error Status Code

What is the expected correct behavior?

Login via my.typo3.org works, session is created, user is redirected back to regular typo3.org site.

Acceptence Criterias

• frontend login via my.typo3.org must be possible

Relevant logs and/or screenshots

Oops, an error occurred! Event: 3b0513a96ddb463b89e6e0770d368df7
Return value of T3o\T3olayout\Provider\Typo3ResourceOwner::getId() must be of the type int or null, string returned

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)