After logging in to forum or ACP, a white page with error message appears. This error can be bypassed only with OAuth-login.
Catchable fatal error: Argument 6 passed to phpbb\auth\provider\db::__construct() must implement interface Symfony\Component\DependencyInjection\ContainerInterface, string given, called in /path/to/forum/phpbb/auth/provider/oauth/oauth.php on line 163 and defined in /path/to/forum/phpbb/auth/provider/db.php on line 48
(only sid value is left out)
I believe this bug is related to OAuth, and perhaps different logins from two browsers. I'll give detailed explanation what I did.
Laptop: I had previously created session ("remember me"), with username and password.
Phone: I logged in with OAuth (facebook).
Laptop: I tried to login to ACP, but faced the error page. I used browser's back-button to get back to forum view. I logged out from forum, and was unable to relogin (username + password), as I was offered the error page after posting login-form.
Phone: I logged out.
Laptop: Retrying with logging in with username and password did not work, but OAuth (facebook) did work. After this, logging in to ACP still fails.
Currently I'm unable to login with either of my accounts by using username and password; founder-account and admin account - latter is the "main" account, which also is used in above explanation.