I've got the same issue. The error occurs random, even if my macbook was not in standby. Enpass "forgets" all entries over all 3 vaults (synced via webdav).
OS Version: mac OS Big Sur 11.2.3 (20D91)
Enpass Version: 6.6.0 (772) (via Mac App Store)
Hardware: MacBook Pro (13-inch, M1, 2020)