We switched from SOGo version 2.3.23 to version 4.3.0.
For that we made a complete backup with sogo-tool on the 2.3.23 machine, copied that backup to the new 4.3.0 machine and made a sogo-tool restore.

This was processed without errors.
Afterwards some users can log in successfully, but can only see a white screen or error 501 (depending on used browser).
In sogo.log we see an 501 error about PreventInvitationsWhitelist.
When checking that for "affectedUser" with
/usr/sbin/sogo-tool user-preferences get settings affectedUser Calendar
we get
Calendar: {"PreventInvitationsWhitelist": "", "PreventInvitations": 0, "View": "weekview"}

As you can see PreventInvitationsWhitelist is stored as empty String, but is defined as array.
Fix is to set it to an array with:
/usr/sbin/sogo-tool user-preferences set settings affectedUser Calendar '{"Calendar":{"PreventInvitations":0,"PreventInvitationsWhitelist":{},"View":"weekview"}}'

I checked all affected users.
They all are very old, which means they were migrated from SOGo version 1 to SOGo version 2 to SOGo version 4.
In their backup file that wrong setting was stored as empty String.
And sogo-tool V4 imported it as empty String without giving any error.
But it also did backup it again without error as empty String!

sogo-tool should either convert that empty String to an empty array on restore, or should give an error on setting, backup and restore.