Cannot add product in backend

-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.4.3
-- PHP version -- : 5.4.4
-- Browser(s) name and version -- : firefox 40.0.3

I haven't added any products for quite a while.

Now when I try to add something I get this message:
Error
Please complete the Frequency field

This is a custom field, set to 'not required'.

I need to add products asap, so this is not a good time for problems that did not exist a while back. I assume it must have something to do with the latest updates of Hika Shop?
Cheers

Just sent you a PM with login details.
Cheers

Now this is weird...

I just wanted to show the effect to someone else - and now it's working.
The only difference to last time's attempt (after downloading the complete installation package) is, I've restarted my computer in the meantime. I have no clue what's going on here...
Cheers