Sounds like update got lost. See About → System info BaseVersionName and ServerVersionName.
Base is the initial install, maybe from Asus. Update is stored separately, and can get lost if corrupted. Corruption detection is pretty sensitive, so even writing a file into an update can get it declared “bad”. Deleting it completely (if Asus does so) would of course also remove it. Suggest you look for updates.
My Linux Mint (Ubuntu base) put base in /usr/lib/duplicati, and updates in /usr/share/Duplicati/updates/, however we’ve seen that Asustor sometimes uses a different folder layout. One way to look is using ps.
Depending on what ps you have, there are probably options (sometimes -ef) to see Duplicati’s options.
Base:
184:root 25692 1 0 Nov17 ? 00:00:12 DuplicatiServer /usr/lib/duplicati/Duplicati.Server.exe --webservice-interface=loopback
Latest update:
166:root 16964 25692 0 09:23 ? 00:00:04 /usr/bin/mono-sgen /usr/share/Duplicati/updates/2.0.7.2/Duplicati.Server.exe --webservice-interface=loopback
EDIT:
So once you find your update, then see if it’s still there after the restart – with no changes done inside.