Is it possible this happens when you daily SQL backup is running?
If you are using mysql, make sure to use the
it will not slow down the availability of the database.
I combine with --single-transaction --routines
If you never used --single-transaction before, this problem would grow over time, as your DB gets bigger.
Two ways your can get client devices asking for password again:
The Z-Push is available, but kopano-server is not available
If the database server is too slow to accept new requests (or not available/offline), kopano-server can’t do much in regards to authenticating users.