Hi vmtatze,
RJIn is correct: What you are seeing results from an incompatiblity between the Mountain Lion guest OS and the VMware Tools in Fusion 4.x. There was quite a bit of work needed to get VMware Tools functioning correctly inside a Mountain Lion guest, and Fusion 5 is the first version where that guest OS is supported.
Unfortunately, installing Fusion 5 will not automatically fix the problem for your existing VM: You will also need to remove the old VMware Tools from inside the guest to fix the crash, and to do that you will probably need to use Safe Mode inside the guest. If you have a recent snapshot of the VM from before you installed VMware Tools, it is probably easier just to restore that. Let us know if you need help getting your existing VM going again!
Cheers,
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Darius