as WalterH already states well:
auth_backend_selection=allow_all
for example in /etc/gromox/imap/authmgr.cfg
, you read correctly, the dir imap does not exist per default, so create it. for details check https://docs.grommunio.com/man/gromox.7.html#listing-of-config-files-per-component
or simply run another gromox-imap process on a different port with auth_backend_selection=allow_all cfg on a different port.
(you obviously should not be running that unsecured imap ever publicly available, so make sure you secure the imap during the migration)