Category list not available in virtuemart 2.6.6.

[jelly]

Yesterday we noticed that we were no longer able to select a category for a newly created product.

In case of the already existing products, the list with categories shows like it is supposed to.

The problem is quite similar to this old topic: forum.virtuemart.net/index.php?topic=53008.msg172555#msg172555

However, there it concerns an older VM version. Moreover we have no >200 categories, we  have 57 categories. The problem occurred at creating product ID 600, don't know if this is related to the problem...

So... we are not able to add products anymore, which is quite a big showstopper here...

Virtuemart 2.6.6.
Joomla 2.5.28
Extra info: If I could, I would ofcourse upgrade to newer versions. I am not able to upgrade to newer versions because of our paymentprovider which is not ready yet for the newers versions of VM en J, but I do not directly think this has got to do with my current problems since the discrepancy of our paymentprovider is already going on for some months now. :S

HOPE SOMEONE CAN HELP US HERE!