When you are unable to login to the unifi controller or forgot admin password, you can restore access using SSH and manipulating mongodb directly.

Do not uninstall unifi controller - most of the data is not stored in mongodb. In case you thought a mongodb backup would be sufficient, you may have fucked up already, just like me. However I managed to write this "tutorial" for anyone to not run into the same trap.

Apparently this guide no longer works with recent unifi controller versions (starting nov/dec 2022). Since I no longer use unifi hardware in my home system, I can not update the guide myself. In case you've gotten here to recover your data, you're likely doomed. But giving it a try won't hurt anyway, therefore: good luck.