Error running ASPX test page

[gtilles]

I have site that I did in Classic ASP 5 years ago, the owner now wants to make some changes to the code. I decided to see if moving to .net was an option since the server claims to support .net.
I created a default project and created a page called WebForm1.aspx
I dragged a button to my form and in the button click put a response.write "hello"
Simple enough....
I ftp'd the bin dir with the .dll file as well as the Webform1.aspx page to the server.

I get the following error. Yet I uploaded the same files to 2 other different hosting company servers that belong to other clients and everything works as intended.
They called me back and said there was a problem with my code (typical host response) and the codebehind was pointing to a file that doesn't exist.
How is it that it works elsewhere but not on their server?
What steps can I do to troubleshoot on my end more or should I just wait to see what they find?

Server Error in '/' Application.
--------------------------------------------------------------------------------
 
Runtime Error 
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine. 
 
Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off".
 
 
<!-- Web.Config Configuration File -->
 
<configuration>
    <system.web>
        <customErrors mode="Off"/>
    </system.web>
</configuration>
 
 
Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL.
 
 
<!-- Web.Config Configuration File -->
 
<configuration>
    <system.web>
        <customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
    </system.web>
</configuration>