I had figured out the same issue. Suddenly Error 500 on Cyberpanel Admin URL :8090
But not on .8090/phpmyadmin
Last thing I did was a Ubuntu system update with
apt dist-upgrade
one day before.
Becaue I had a snapshot from before my update I experimented a bit and could fix this issue in my case by doing an cabyerpanel Upgrade to the same version with the official way.
I belive my database was updated and some cyberpanel specific settings got lost.
So my advice; Do a “upgrade” of cyberpanel installation to the same version.