Hey Nils,
When you say it happens on all the 4 Mac(s), does all the 4 Mac(s) run the same applications? This isolates that there is something similar is happning in all the 4 Mac(s) could be the application itself(Just being skeptical )
-Is that the Windows freezes or it the virtual machine on Fusion freezes?
-If you feel that its from Windows did get a chance to have the event log to see any errors.
-If you feel you may also upload the VMware support logs. To collect the best support logs when the error occurs and just after you recover from the error, go to Help in the in the VMware Fusion menu from the top menu bar >Select collect support information. This will create a tar file on your Mac desktop.