VM 3.0.18.6 BE edit variant

[diri]

When trying to edit a variant there is a redirect to new window which shows start page of virtuemart in backend and a message:

Error
COM_VIRTUEMART_ALERTNOTAUTHOR

In general it looks like editing variants is simply impossible (user is superuser).

Tried to add a new variant - doesn't work as well. Deletion of a variant works.
edit:
There has been one parent product with two variants. It worked well with one difference between those variants:
One variant has been assigned to a category. Therefore it was shown in category view - that's not really correct.
Tried to edit it - no go. Deleted it and created a new variaton via stockable fields. Result is somewhat obscure, see snippet attached please.


Any hint?

Thank you,

diri
VM 3.0.18.6
Joomla 3.6.5
Firefox 50.1.0

[diri]

Ok, now at VM 3.0.18.7:
Frontend category is shown correct as well as product detail.

The deleted variant (the variant with category, see previous post, please) still exists in backend.

Found a possible cause for not being able to edit variants in backend:
I enabled multi-vendor.

Nevertheless I should be able to edit a variant as superuser as well as being the vendor of the product. Whenever a vendor can't edit his products it's useless.