Another possibility is the need for browser hard refresh that might be causing other issues e.g. here.
The previous JavaScript could only display LastDuration, and new design uses LastBackupDuration.
The theory seems weak because old JavaScript looks like it said “Last successful run” (not backup).
Hearing back with some history might help, e.g. is this just after upgrade or have backups been run?
I’m also still interested in seeing which variables are in the Duplicati-server.sqlite Metadata table, e.g:
BackupID Name Value
20 LastBackupDuration 00:00:25.4285180