Variants stopped working in backend

-- HikaShop version -- : 6.4.1
-- Joomla version -- : 5.4.6
-- PHP version -- : 8.4.21
-- Browser(s) name and version -- : Edge
-- Error-message(debug-mod must be tuned on) -- : No error messages

We have a few product and have used variants for some time
But suddenly we can't edit variants on existing products or add variants to any products
After we have chosen charatistics an go to product and chose variants, we can click on "add", but we can't do or chose anything there
A "box" comes up, but nothing works there

Can you help?
Thanks

Finn