We have following problem. Someone seen this before and know solution?
This happens with PCS7 6.1 SP3 and also with 7.0 SP1 and 7.0 SP3.

When both servers are running – all clients are looking at the main server OS1.
We stop the wincc runtime on server OS2 (redundant) to do maintenance, everything is fine, clients are working fine.
After restart of server OS2, we start the wincc runtime on it and stop the runtime on server OS1 to do maintenance on it also. Clients switch to server OS2, data from PLC are available but you cannot switch between the process screens, cannot open dialog windows, cannot press buttons, etc. So you can see that process values are changing, but all wincc interface frozen.

You can solve this issue the following way:
1.Reboot server OS1, wait when runtime is running;
2.Stop runtime on server OS2, clients are looking at OS1 again now operators can switch screens – everything is working;
3.Reboot server OS2, wait runtime is running
4.Stop runtime on server OS1, clients are looking at OS2, operators still can switch screens – everything is working.