SendToModule doesn't work, when Pointer is grabbed in a VM

I have written a nice mpd-control for the FvwmButton.

Inside the FvwmScript which shows informations about the actual song is a part which changes the icons for play/pause and the volume-button. This Buttons are above the swallowed script and use mpc to control the mpd.
I have a function defined to change the Icons of the control-Buttons in FvwmButton.

The function is:

+ I SendToModule FvwmShelf ChangeButton $0 Icon $1

For testing-purposes i start a virtual Machine and control the mpd running on the host from the guest in the VM.
When i send a Command with mpc to the running mpd from the VM, and the Mouse-Pointe isn’t grabbed in the VM, this function above changes the icon correctly.
But if the VM grabbed the Mouse-Pointer i get an error-message

[fvwm][ComplexFunction]: <<ERROR>> Grab failed in function CHANGE_ICON mpdVol audio-volume-muted.png, unable to execute immediate action

Should i live with this, or is there a solution for grabbing the focus-policy?