If you have been working with enterprise portal development, you will probably have gone through the same levels of frustration as I because error messages are not good in adding usable information into these messages. Over time I have collected some sites with suggestions on how to troubleshoot these more or less cryptric messages.
Error: Enterprise Portal Web Parts show "Unable to render content" error
Error: Unable to render content due to an error. Please contact your system administrator.
Generel troubleshooting: https://msdn.microsoft.com/en-us/library/cc554278.aspx