I recently upgraded from 2.5.0 to 2.7 on CentOS and most everything seems to be working OK except that we can't add problems to a problem set.
I log into a course and then select Library Browser. I select a HW set in the "Add problems to Target Set:", pick a Subject and Chapter and click View Problems (from the National Problem Library). When I click "Add" to add a problem to the HW set, nothing happens. I have tried it on multiple browsers (Chrome, Firefox, Safari).
After I click the "Add" button, it does highlight but nothing happens. All of the other buttons seem to work OK except for "Add" and "Add all".
This is one of 3 known common causes of this behavior. Glad it's working for you. Thanks for reporting back.
Another thing that can cause this on an intermittent basis is if you are idle for a long time. WeBWorK will time out your login but the call back for adding problems isn't (yet) smart enough to tell you what happened so it just fails.