I was seeing

:0 read() failed on a timer FD. Error was: (9) Bad file descriptor

after the FS went to read-only mode after some issues with iscsi on my Synology The Unifi webconsole didn't work anymore. After some searching I suspected a db problem. so I attached to the container (I am using ryansch/unifi-rpi) and executed the following:

mongo --port 27117
use ace
db.repairDatabase()
exit

And the interface started working again!