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

atkkierratys(dåt)com/coc/forkku/adm/index.php?sid=
(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.

[edit]
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.