Won't add custom field to product

[EivindJ]

Hello,

For some reason I suddenly can't add any custom fields to products. Nothing happens when I choose a custom field from the drop down menu.

I get an error message like this, which duplicates every time I try to add a field:
Failed to load resource: the server responded with a status of 500 (Internal Server Error) http://mysite.com/administrator/index.php?option=com_virtuemart&view=product&task=getData&format=json&type=fields&id=8&row=6&virtuemart_product_id=495&_=1393488726491

I run Joomla! 2.5.18 and Virtuemart 2.0.26d.

I've newly installed VP One Page Checkout plugin, but disabling this doesn't make any difference.

Any ideas?

[EivindJ]

I figured it out, in one way, but the solution gives no meaning to me. I added a "exit();" php tag at line 179 in /administrator/components/com_virtuemart/views/product/view.json.php, at the very end of the display function. Now it suddenly works. Can anyone explain why?