Hi, thanks for suggestions.
No, it is not a Virtual-System, but a problem with memory shortness might be a good tip. The machine has 32 GB Ram, but it has to run a lot of memory intensive software. So far, I was convinced, that Windows is able to take care of problems like this (swap file,...). When a memory problem could be the cause of this behavior, at least I have a place to look for problems.
Thanks again and best regards