Hi Burkhard ,
Your getting the error because by default snapshots are stored in the location of your vmx. its a block size limitation
KB1091: File is larger than the maximum size supported by datastore
VMware KB: Creating snapshots in a different location than default virtual machine directory
Now if you want to move the VM to different datastore -- depending on license if you have storage vmotion then right click -- migrate -- change data store.
else you may need to power off the VM and do the same migration.
let me know if you require more clarifications.
Thanks,
Avinash