“Sorry! product or inventory is not found” – Error on variable product

Since a few days it’s not possible to book a variable product (try the Link).
I searched for the same Error-message but found nothing.
The last reservation on tis site has been done at the 7th of november.
I tried really everything. I set up the inventory and also the parent-inventory of the variable product.
None of the variable product is working like before.
Could anybody help me figuring out what the problem is?

The page I need help with: [log in to see the link]