The EP is a place where sometime the errors that occur are not that straightforward, visible on the page or saying what is wrong ...
So in this case, here are a few things you can try:
1. of course, first one, make sure you have debug enabled (in web.config debug set to true, stacktrace set to true, and customErrors set to Off);
The detailed steps for the actions above are as follows:
a) First you need to navigate to your web.config file for your EP site. This is normally found at C:\Inetpub\wwwroot\wss\VirtualDirectories\normally_your_EP_site_port_number\;
b) make a copy of the web.config file;
c) replace the line <compilation batch="false" debug="false"> with <compilation batch="false" debug="true">
d) replace the line <customErrors mode="On" /> with <customErrors mode="Off" />
e) replace the line <SafeMode MaxControls="200" CallStack="false" DirectFileDependencies="10" TotalFileDependencies="250" AllowPageLevelTrace="false"> with <SafeMode MaxControls="200" CallStack="true" DirectFileDependencies="10" TotalFileDependencies="250" AllowPageLevelTrace="false">
2. the errors can be seen immediately in the page and point you in the right direction;
3. next, look at the Windows Event Viewer; most of the times the errors are also logged there;
4. in you are still unsure, one thing you can try, if the error is thrown on the page as a pop-up message, try to reload (F5) the page again; you may discover sometime that an AX (more friendly) error is being displayed in the page (in the form of the yellow bar) ...
Hope this helps ...