I just updated to 6.2.4, and I'm still getting this bug.

 

I had this same issue running 6.2.3. What worked for me was to synch up the dnn_user table with the aspnet_xxxxx tables. These include aspnet_users, and others with that prefix. You have to be careful since the user ids are using guids, but it was easy enough because of the user names. Once I did that I was able to add roles again. The reason they weren't in synch in my case was because I updated the usernames to windows ids, since we are now able to use windows auth using the ad module.