After going through the Github thread above I seem to have found a way around this on a Mac, at least for now.
I deleted all the files in ~/.config/Duplicati/ except backup_xxxxxx.sqlite (my latest backup), which allowed Duplicati to start but without any configured backups.
Closed Duplicati, opened backup_xxxxxx.sqlite in TextEdit and copied the contents into the newly created Duplicati-server.sqlite file. Started Duplicati again, repaired the database and things seem to be back to normal.
Thanks @Monstergerm for linking my thread.