We are replacing our old Windows 2008 Active Directory server with a new 2012 R2 one. Our ZENworks 11.4.2 installation has Active Directory/LDAP as a user source, it was pointed to the 2008 server, but since he is going with retirement soon I changed the Users source to the new 2012 server. At first it seems to work fine, but the 2008 and the 2012 are both still running. When I shutdown the old 2008 server, the user source in ZENwork becomes slow, it works but very slow, causing login timout issues with the ZENworks agents on the clients. When I turn the old 2008 back on, it works fast again. This while the user source is configured to use the new server, no links to the old server. I have already tried running a LDAP client directly to the new server, it's fast, no slow responses.

Any ideas what could be causing this issue?