QNAP Domain Controller - Error creating new Domain User

[Tech3996]

TS-453D 8GB Firmware V5.0.0.1932 (29/1/2022)

Have enabled the Domain Controller on this NAS. NAS is called 'server1'. Domain is called 'FM.local'
Domain Users & Groups successfully added.
Windows Client PCs successfully connect as domain clients to 'server1\sharedfoldername' using 'FM\username'.

Now having issue creating a new domain user. The new user name appears, but the domain 'home' folder is not created & an error is logged.
Also having issues with adding existing Domain Users to existing Domain Groups. It's inconsistent.

Control Panel > Privilege > Domain Controller > Users > Create > Create a User> Username Testuser1 > Next > User Group (Selected one existing group)
Error window pops up. "Create a User. The user could not be created. Please check event logs of details."
QuLog Center: "[Domain Controller] Failed to add user "Testuser1" to group "Domain Users".
Domain Controller > Users: "Testuser1" then appears in the Domain Users list.
FileStation: Folder server1\homes\Domain=FM does not contain a folder called "testuser1".

Hard to pin down, but sometimes a specific Domain Group will have all existing Domain Users randomly removed after a period of them being enabled & Window Client PC mapped drives working fine.

[Tech3996]

Ok, so I now know that the 'home' folder is not created until the new user connects for the first time. So I mapped the home folder from a Windows Client & the home folder was created & it connected.

However...

Still having issues with Domain Users being strangely removed from Domain Groups.

I attempted to add a Domain User to an existing Domain Group & got an error. After that, all previously listed Domain Users have been removed from the Domain Group.
Pop-up error window: "Edit Group Users. The users can not be modified. Please check event logs for details."
Log shows: [Domain Controller] Failed to edit the members of group "Production"