A quick check with Google turn up this:

The Web server (running the Web Site) encountered an unexpected condition that prevented it from fulfilling the request by the client (e.g. your Web browser or our CheckUpDown robot) for access to the requested URL.

This is a 'catch-all' error generated by the Web server. Basically something has gone wrong, but the server can not be more specific about the error condition in its response to the client. In addition to the 500 error notified back to the client, the Web server should generate some kind of internal error log which gives more details of what went wrong. It is up to the operators of the Web server site to locate and analyse these logs.

This is a long way of saying that the problem is not with your code, dear Brutus, but with in your Server. (My apologies to Bill Shakespeare, but it is waaaay before coffee.)