Child products no longer displayed

[chaldama]

Child products which used to display fine are now not displayed anymore. Instead of the dropdown with child products, only the pink box is shown, please see: http://morlie.nl/bestel-kaarten/zwart-wit-condoleance/kaart-001-detail
This could have happened after updating to 3.0.18 but perhaps it already went wrong in 3.0.16 and we just didn't notice before.
Joomla is running on 3.6.2, PHP 5.6.22.
Changing to Joomla's default template did not make a difference, nor did disabling all extenions except VM.

[GJC Web Design]

Normally it would be a template problem but if u say they show on Protostar then ...  deeper investigation needed

there is simply no code in the pink box.. 

[chaldama]

Ok thank you for checking the issue, what would you suggest to do now?
If I change to Protostar, there is no box at all.

[GJC Web Design]

I don't now.. the code, install, config etc etc needs to be looked at..

[chaldama]

Do you need a login to check this? Or would you like me to make printscreens?
If so please indicate which screens you would like to see.

[GJC Web Design]

sorry.. no house calls unless u hire me...

[chaldama]

Ok no problem. I will just have to discuss this with my customer, what will be the costs?

[GJC Web Design]

email please...

[chaldama]

You've got mail ;-)

[chaldama]

Problem solved, thank you.
For others that experience problems with child products: it was a problem with the Breakdesign Stockable variant plugin, not with the Virtuemart core multivariants.