What version of Adaxes are you using (including the build number)? Also what browser and browser version are you using?
When the issue occurs, do you do anything else on the same page besides selecting the users? Do you go straight to the Members section, start selecting the users and the issue occurs, or do you do anything else before selecting the users when the issue occurs?
Such behavior is very strange and we didn't see anything like this before, so we need as much info from you as possible in order to reproduce it in our testing environment.