Can't create New User in WordPress

Hi everyone,

I went through the forum but couldn’t find a solution for my issue:

I recently added AffiliateWP to my wordpress site. In order to create a profile for an affiliate I need to add him as a user to the page first. Unfortunately that is not possible. I put in all the infos and save it (no error message or anything strange). The user just doesn’t show up but only my own administrator user is there. The weird thing is that from the first try on there is a blank white line that looks like a user underneath. I have been in touch with AffiliateWP and they logged in and checked but couldn’t find anything either but ment it was a wordpress issue.
Has anyone ever had that problem?

thank you so much!