Cannot add product variant

I cannot add a product variant.

Steps to reproduce:

1. Open an existing product with variants.
2. Add another option value (i.e. size).
3. click re-generate variants and confirm with yes.
4. nothing happens
5. click save
6. after reload your variants are removed. the variant list is empty.
7. click generate variants

Expected result:
Variants should be recreated

Actual result:
nothing happens

This is on a live site - I really need a fix soon.

Thanks!