Login in Webinterface stopped working

What is the problem?

I do have a Rasperry PI 3 with OctoPi and a Prusa Mini, that I bought a couple years ago as a finished set-up in an enclosure. I use it rarely, and today I needed to print something. After booting up the system, the login screen in the webbrowser (http://octopi.local/) appears as always, but after entering the correct credentials, the login gets stuck on a white screen showing " Connecting to OctoPrint's server..." and a rotating circle formation below it.

Sending a print file to the printer from PrusaSlicer is working, also I can operate the printer from my phone via the OctoPod app and it is currently printing fine, but still no login is possible.

What did you already try to solve it?

I restarted the PI multiple times and also disconnected the printer before starting the PI. I removed the SD Card and checked it in a PC and copied everything I could copy, which was limited to the "boot" partition, not sure if something relevant is in there.

I tried connecting to it via SSH, and while the connection worked fine, I realised that my user does not have SSH rights and I cannot find the original login the seller gave me.

I also connected a screen via HDMI to the PI, everything seems to load fine and it ends up with a login screen, where I again cannot login with my user.

Have you tried running in safe mode?

No, since I found no way to do so

Did running in safe mode solve the problem?

Not applicable

Now I am wondering if my only way is to do a fresh install, which I am trying to avoid without a proper backup since there is a custom LED stripe, two temperature sensors and an enclosure fan installed, controlled by the PI and I do not have any sort of documentation. Also, the seller claims not to have any old documents incl. logins.

I know I should have done proper backups and checked the whole set-up incl. SSH and local login while it was still running...does anyone of you have an idea how to get access to the PI or do some sort of backup from the SD card? Otherwise, I guess I have to do a fresh install and figure out the hardware details on the way.

Thanks a lot for your help.