I am sure that the problem is due to an error that I introduced when I was modifying the topic questions. Does anyone have any hints as to how I can find out where the problem occurs?
Here's the warning message that gets displayed.
WeBWorK has encountered warnings while processing your request. If this occured when viewing a problem, it was likely caused by an error or ambiguity in that problem. Otherwise, it may indicate a problem with the WeBWorK system itself. If you are a student, report these warnings to your professor to have them corrected. If you are a professor, please consult the warning output below for more information.
Argument "" isn't numeric in numeric gt (>) at /users/amstaff/v0ww/webwork/webwork2/lib/WeBWorK/ContentGenerator/GatewayQuiz.pm line 816.