Folder redirection to a share on a server in a different domain

Hello,

I need some help to solve a folder redirection problem to a share in a different domain.

Using folder redirections via a loopback gpo for remote desktop connections did work before without a problem when both domains where running Windows Server 2008 R2 domain controllers. The users were manually created on both domains with the same passwords.

Now after upgrading one domain to Windows Server 2012 R2 the folder redirection is working for one direction only.

Before:

Domain1 = Windows Server 2008 R2 (2 ADs, both 2008 R2)
Domain2 = Windows Server 2008 R2 (2 ADs, both 2008 R2)

Login from domain1 to a RDP server in domain2 did redirect the documents folder to a share on fileserver1 in domain1, logged in with "domain2\username".
It did work for both directions.

Now:

Domain1 = Windows Server 2008 R2 (3 ADs, two 2008R2, one 2012R2)
Domain2 =...