Can you add this key in the web.config:
<add key="CMSDisplayApplicationErrorMessages" value="true" />
Defines the level of information which is displayed to the user if the application fails to initialize (e.g., the application fails to connect to the SQL server):
false - only general error message is displayed.
true - detailed information about the origin of the error is displayed.
Also check for:
<system.web>
<customErrors mode="Off" />
</system.web>