Hi Geertjan,
I have only personal experience with this problem. It feels like the input is waiting for something [GUI] specific, but didn’t get it. In the moment I hold down the key “1” and move the mouse or pen—the system seems to be satisfied, and works nicely again. Quick and easy.
Since I have a similar behavior in Photoshop from time to time, I searched elsewhere as well. So far I have only suspicions—as the behavior is not constant. So, my idea was that an input device [e.g., tablet] might be the cause.
However, the way that I have to reply here is, to suggest the support, as it is not an education theme, it falls under technical problems. I’m not supposed to handle this. The option to use the support is preferable in this case for many reasons, they work in a system that is very communicative with the developers, and it collects all data in a central way. If if I would be able at all to fix it here, it would be not a help to the community.
Please check if they have already an idea for you:
http://www.maxon.net/support/support-questions.html
All the best