Hello MarjanP,
I would recommend to check the registry of one of those users, as well the machine applicable policy.
In the registry, you can check the next keys. Value should be "0" for none hidden drives (this might be a workaround for users in OU2, by modifiying the registry key by GPO on logon)
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]
[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]
Also, restore any Local policy if any using the next command from elevated command promtp:
secedit /configure /cfg %windir%\inf\defltbase.inf /db defltbase.sdb /verbose
then reset downloaded policies
RD /S /Q "%WinDir%\System32\GroupPolicyUsers"
RD /S /Q "%WinDir%\System32\GroupPolicy"
gpupdate /force
Hope this helps with your issue,
Best regards,