Dear olga is clear what the message says. Turn off the custom errors to see the details about this error for fix it, and then you close them back. So add mode="off" as:
<configuration>
<system.web>
<customErrors mode="Off"/>
</system.web>
</configuration>
Relative answer: Deploying website: 500 - Internal server error
By the way: The error message declare that the web.config is not the one you type it here. Maybe you have forget to upload your web.config ? And remember to close the debug flag on the web.config that you use for online pages.