OK -- I finally solved this by (a) disconnected Enpass from my NextCloud account (b) deleting the entire Enpass folder on my NextCloud server and (c) recreate the connection from Enpass to my NextCloud server (NB using WebDAV as the NextCloud option on Enpass still doesn't work)
This fixed the problem and all my other devices are now syncing again.
So something happened inside that Enpass folder (which nothing touches except the Enpass clients) that corrupted it and prevented Enpass from syncing again.