Strange as it seems but “Server encountered an internal error. For more information, turn on customErrors in the server’s .config file.” is actually begging for you to add,

<system.runtime.remoting>
    <customErrors mode="off"/>
</system.runtime.remoting> 

Thanks go to Chris Taylor for pointing this out. I didn't think it ever seemed to work properly before!

http://dotnetjunkies.com/WebLog/chris.taylor/articles/5566.aspx