Ubuntu 16.04 is based on Debian Stretch. Pi-hole is incompatible with this version since mid-2022. Your Pi-hole must be a very old version which was compatible with this OS. Overriding the OS check means, I think, that some incompatible components may now be present.
The ideal approach would therefore be to move to a supported current version of the OS. Do you have a current Teleporter backup you can use with a new install to restore your lists, etc? This can be created via Settings > Teleporter > Backup or with the command pihole -a -t.
However, since your machine is in this state where the web server won't load, if the command line doesn't work, a dev may be able to advise on how to get the web server running again so you can perform a backup.