I had the same issue. Lost access after the 6.8.0 RC1 update I did on Oct 12, and finally noticed it today. I had this error in my Unifi log file, and nothing else, for the past 18 days:
[2019-10-12 13:46:15,023] <db-server> ERROR system - [exec] error, rc=134
Container logs showed nothing interesting. I was using the "Bridge" network mode.
I ended up fixing it, but I'm not sure exactly what did it. I did a few things:
Since the log was showing a db error, I did a MongoDB database repair with these instructions. This seemed to have no effect.
Change network type from Bridge to Host, didn't work.
Lots of container restarts and force updates.
Following something I read here when investigating the big "sqlite corruption" issue a while back, I changed the config folder mapping from /mnt/user/appdata/unifi-controller to /mnt/cache/appdata/unifi-controller. I think this may have done it, but I don't want to break it again to check.
I also changed from Host to br0, then back to Bridge. I don't think this did anything, but I did it at the same time as the previous.
I don't have Wireguard set up, so it's probably not that.