It seems it works fine now. Yesterday we had global problems with PHP on server, they were not related to your plugin. Maybe after fixing them your plugin began to work too. Anyway thanks for your great help..
INTERNAL SERVER ERROR
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the administrators and inform them of the time the error occurred, and anything you might have done that may have caused the error.
When I try to add a question, page reloads and I get error message. But when I go to questions list I can see that the question has been added successfully .
Forum is on Windows hosting. Where is the problem?