Central giving login/access issues

Hi,

Central version 1.3.3

ODK Central has started giving login issues, from both browser and mobile app.

In browser, when I try to login from admin account, it is saying this:

A user is already logged in. Please refresh the page to continue.

And from non-admin account, I get this message:

Something went wrong: error code 500.

The enumerators are facing following issue when trying to submit the data:

I have checked the server in detail. Docker is fully up, there is no issue on either disk space or server memory/RAM. The login page of central opens up fine, but it does not let me get past it. It does not even throw regular INVALID PASSWORD error if I give a wrong password. Just same old error code 500.

Can someone please help in troubleshooting?

Many thanks,
Saad

Just to add: The server and ODK Central was running fine since over a month, with around 2000+ submissions done from the field with no issues. Even earlier today I logged in and it was working OK.

This is the abnormal line i see in nginx logs:

39.41.188.114 - - [15/Feb/2022:20:46:11 +0000] "GET /v1/sessions/restore HTTP/1.1" 500 271 "https://boreholes.govtech.solutions/" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/98.0.4758.82 Safari/537.36"