The error was found when i try to open an uploaded .txt file.
The message was as follows
The source code that generated this unhandled exception can only be shown when compiled in debug mode. To enable this, please follow one of the below steps, then request the URL:

1. Add a "Debug=true" directive at the top of the file that generated the error. Example:

<%@ Page Language="" Debug="true" %>

or:

2) Add the following section to the configuration file of your application:

<configuration>
<system.web>
<compilation debug="true"></compilation>
</system.web>
</configuration>