This is easy to fix but you have to have access to the server to do so. Supposedly John Jones has submitted a fix (probably in WW 2.8) so that people will not be able to enter invalid time zones. In the mean time, please make sure you enter only valid timezones (click on the info icon).
To fix this go to the course directory (e.g. /opt/webwork/courses/my_course)
and delete the file "simple.conf". Actually if you have done other coure configuration you can edit simple.conf removing the timezone line.