Thanks a lot for your help!
Yes, that's a test I requested a few hours ago. I'm still waiting for a result; since the department uses Citrix only and I do not have access to the users, it might take a while to locate a workstation and make the users try it from there.
We are going to test against IE10 using Citrix and on an ordinary workstation, too, as Christian suggested, but this will take even more time since this is not our standard environment and we'll have to set up a special terminal server and a new workstation profile for that. Maybe we'll even manage to get Firefox onto a workstation to test that, although this might violate our security policies. And I cannot test myself, obviously, since I cannot reproduce the error.
Oh, and IE Compatibility Mode is turned off by default, so I'd guess that the users are not using it, but this still has to be verified.
I'll keep you informed.
Michael