Extremely slow logon to ICA/RDP showing "Checking your credentials..."

Together with two of my collegues I recently worked on a Citrix problem with extremely slow logon. When trying to logon with ICA the server showed "Checking your credentials..." for a very long time (maybe 15 minutes) and RDP just gave me a blue background approximately the same time (after entering a valid ID/PWD).

After cheching verious things like:

• NETLOGON service (both on Citrix server and domain controllers in the same site)
• KDC service (on the DC's)
• Bindings on the NIC (Citrix server)
• IMA service (on the Citrix server)
• Time (i.e. no time skew difference on the server, for Kerberos to work)
• ICA connection/protocoll settings
• A load of other possible things

My collegue (Jostein Støen)found the issue to be a DC (Windows 2008 server) that had cleaned a file with a virus and required a restart to fix all errors. After the restart all worked well again. The slow logon was caused by the server session timing out to the first DC and after the timeout the logon succeeded as normal.