It would be nice to get some more diagnostic information on the error message page. So in Internet Explorer you can do this: Tools | Internet Options | Advanced | Uncheck Show friendly HTTP error messages. And it doesn't hurt anything to just keep your browser set like this.
And then when you get an error you can see more on the error page especially the code at the bottom which could be something like "Error on line 339 GetValue()) extra parenthesis"?