If you cannot retrieve the error from the Control Panel's Exceptions Log, you can disable friendly error messages by editing the web.config file. To collect the error by disabling friendly error messages,
1.Edit the web.config file in the root of your site
2.Edit the <customErrors> node in the web.config file and change mode="RemoteOnly" to mode="Off". Be sure to match the text case shown here ("off" is not equivelent to "Off")
3.Save the web.config file.
4.Reproduce the error by accessing the site.
The underlying error message should now be shown when the error is reproduced. When you have reproduced the error, you may want to restore mode="RemoteOnly" in the web.config file.