If messages like this appear in your logs this might have to do with (long running) database tasks:
E-2207: /var/lib/gromox/user/2/1: could not obtain exclusive access within 60000000000 ns (mailbox busy with <>/<>)
...
dbop_sqlite: upgrading /var/lib/gromox/user/2/1/exmdb/exchange.sqlite3 to schema EV-14
dbop_sqlite: Completed upgrade of /var/lib/gromox/user/2/1/exmdb/exchange.sqlite3 in 92.47s.
E-2207: /var/lib/gromox/user/0/1: could not obtain exclusive access within 60000000000 ns (mailbox busy with <>/<>)
...
dbop_sqlite: upgrading /var/lib/gromox/user/0/1/exmdb/exchange.sqlite3 to schema EV-14
dbop_sqlite: Completed upgrade of /var/lib/gromox/user/0/1/exmdb/exchange.sqlite3 in 401.06s.
E-2207: /var/lib/gromox/user/1/3: could not obtain exclusive access within 60000000000 ns (mailbox busy with <>/<>)
...
dbop_sqlite: upgrading /var/lib/gromox/user/1/3/exmdb/exchange.sqlite3 to schema EV-14
dbop_sqlite: Completed upgrade of /var/lib/gromox/user/1/3/exmdb/exchange.sqlite3 in 599.77s.