When administrating websites with the Joomla Toolkit by Plesk, Virtuemart will stopp Joomla Instances to work with the Toolkit.
The Problem is the call:
class plgSystemVmLoaderPluginUpdate extends JPlugin {
in Line 22 in the file: vmloaderpluginupdate.php
(Maybe there are more problems with that file...) - well - by deactivating that Plugin the Toolkit and the Joomla - instance will work together again.
But the Checkout process will lead to problems...
So it would be great to have an updated version of that file.