Keyboard "freezes" when sending Windows key combos to host OS

As I shortly mentioned in the "Is it possible to send alt-tab (etc.) to host
Windows?" thread I often have problems that my keyboard freezes in my VPC.
Then I just change the setting to "On guest operating system" and then back
to "On host operating system" and everything works fine for a few hours.

Any ideas what could cause this or hopefully solve it?

TIA
Peter

Am I really the only one with this problem, or am I just the only one using
this setting?

/Peter