Gabrie,
I ran into the exact same issue with a VM I imported, did not experience it with any VM's created inside vCloud. It seemed as if any modification to the VM would throw the error just as you describe. The odd workaround that I found was to set the red threshold very low, below what was used, I did 3GB even though my storage was 2TB. It then allowed me to modify the VM, after that I went back and reset my threshold to the proper value. I have yet to log as support request but this seems like some sort of bug.
-Jason