Looking on MS TechNet I found the following post: Remote configuration failed on WSUS Server.
For me this did the trick:
1) Uninstall WSUS (without database, logfiles, updates)
2) Install WSUS again + KB2720211 + KB2734608 updates
3) Reboot the server (you never know)
4) Start "Synchronize Software Updates"
After that WSUS synchronization is working again and Endpoint Protection is up-to-date on all servers now. Good thing to know when you update your environment to SP1 I think.
Update 8-1-2013: For it seems it's a known issue in SP1 when using specific WSUS ports. Just change the ports back to the ports configured earlier on your Software Update Point and you'll be fine!