This problem is occurring on trunk. I tried to reproduce on the 10.x maintenance branch, but I couldn't add a question to a pool without getting a stack trace.
In a questions pool with 2 or more questions if I attempt to sort on Question Text, I get a stack trace. If you try to sort on Question Type, instead of displaying the sorted results, I am returned to the main Assessments screen.
To reproduce, create a question pool with 2 or more questions. First sort on question text to see the stack trace. Then try sorting on Question Type.
Also, please note that Questions pools seem to be broken completely on https://qa10-mysql.nightly.sakaiproject.org/ in that you cannot even add a question to a pool without getting a stack trace. As a result, I ciuld not test whether the sorting problem was evident there.