[servicestack] 500 Error on AppHarbor but downloaded build works on my machine

I'm using Visual Studio 2013 on Windows 8. I have a web service built off ServiceStack. Everything works fine on my machine but when deploying it to AppHarbor I get a 500 error. I set customErrors mode="off" and I still get a 500 error with no stack trace.

According to AppHarbor's FAQ:

If enabling Custom Errors doesn't produce a stacktrace, then that's indicative of a configuration problem that prevents the ASP.NET runtime from initializing. You can debug such problems by deploying your app (either built locally or build output downloaded from AppHarbor) to a full IIS running on your local machine. You must configure the application pool to run in Integrated Pipeline mode to properly replicate AppHarbor's environment.

So I did this. I downloaded the build output from AppHarbor and ran it on my local IIS and it worked fine!

The AppHarbor Errors page says

No errors to display.

And a AppHarbor's log session feature displays no meaningful information:

2013-12-31T09:55:20.886+00:00 appharbor web.1 Created new worker (version 1388526921) 2013-12-31T09:55:24.864+00:00 appharbor web.1 Warming up (version 1388526921) 2013-12-31T09:55:32.134+00:00 appharbor web.1 Web worker root URL returned HTTP status code 500 (Internal Server Error) (version 1388526921)

Any suggestions?

-- for references, here's an outline of my web.config:

<?xml version="1.0" encoding="utf-8"?> <!--   For more information on how to configure your ASP.NET application, please visit   http://go.microsoft.com/fwlink/?LinkId=169433   --> <configuration>   <configSections>     <sectionGroup name="dotNetOpenAuth" type="DotNetOpenAuth.Configuration.DotNetOpenAuthSection, DotNetOpenAuth">       <section name="openid" type="DotNetOpenAuth.Configuration.OpenIdElement, DotNetOpenAuth" requirePermission="false" allowLocation="true" />       <section name="oauth" type="DotNetOpenAuth.Configuration.OAuthElement, DotNetOpenAuth" requirePermission="false" allowLocation="true" />       <section name="messaging" type="DotNetOpenAuth.Configuration.MessagingElement, DotNetOpenAuth" requirePermission="false" allowLocation="true" />       <section name="reporting" type="DotNetOpenAuth.Configuration.ReportingElement, DotNetOpenAuth" requirePermission="false" allowLocation="true" />     </sectionGroup>   </configSections>   <appSettings>     ....   </appSettings>   <connectionStrings>     ....   </connectionStrings>   <system.web>     <customErrors mode="Off">     </customErrors>     <httpHandlers>       <add path="*" type="ServiceStack.WebHost.Endpoints.ServiceStackHttpHandlerFactory, ServiceStack" verb="*" />     </httpHandlers>     <compilation debug="true" />   </system.web>   <system.webServer>     <validation validateIntegratedModeConfiguration="false" />     <handlers>       <add path="*" name="ServiceStack.Factory" type="ServiceStack.WebHost.Endpoints.ServiceStackHttpHandlerFactory, ServiceStack" verb="*" preCondition="integratedMode" resourceType="Unspecified" allowPathInfo="true" />     </handlers>     <directoryBrowse enabled="true" />   </system.webServer>   <runtime>     <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">       <dependentAssembly>         <assemblyIdentity name="MongoDB.Driver" publicKeyToken="f686731cfb9cc103" culture="neutral" />         <bindingRedirect oldVersion="0.0.0.0-1.8.2.34" newVersion="1.8.2.34" />       </dependentAssembly>       <dependentAssembly>         <assemblyIdentity name="MongoDB.Bson" publicKeyToken="f686731cfb9cc103" culture="neutral" />         <bindingRedirect oldVersion="0.0.0.0-1.8.2.34" newVersion="1.8.2.34" />       </dependentAssembly>     </assemblyBinding>     <!-- This prevents the Windows Event Log from frequently logging that HMAC1 is being used (when the other party needs it). -->     <legacyHMACWarning enabled="0" />   </runtime>   <uri>     <!-- The uri section is necessary to turn on .NET 3.5 support for IDN (international domain names),          which is necessary for OpenID urls with unicode characters in the domain/host name.           It is also required to put the Uri class into RFC 3986 escaping mode, which OpenID and OAuth require. -->     <idn enabled="All" />     <iriParsing enabled="true" />   </uri>   <system.net>     <defaultProxy enabled="true" />     <settings>     </settings>   </system.net>   <dotNetOpenAuth>     <!-- This is an optional configuration section where aspects of dotnetopenauth can be customized. -->     <!-- For a complete set of configuration options see http://www.dotnetopenauth.net/developers/code-snippets/configuration-options/ -->     <openid>       <relyingParty>         <security requireSsl="false">         </security>         <behaviors>           <!-- The following OPTIONAL behavior allows RPs to use SREG only, but be compatible                          with OPs that use Attribute Exchange (in various formats). -->           <add type="DotNetOpenAuth.OpenId.RelyingParty.Behaviors.AXFetchAsSregTransform, DotNetOpenAuth" />         </behaviors>       </relyingParty>     </openid>     <messaging>       <untrustedWebRequest>         <whitelistHosts>         </whitelistHosts>       </untrustedWebRequest>     </messaging>     <!-- Allow DotNetOpenAuth to publish usage statistics to library authors to improve the library. -->     <reporting enabled="true" />   </dotNetOpenAuth> </configuration> 

Just a wild guess: (not much to go on) but I have had similar problems when, for example, I was using the IIS rewrite module on my local machine (and it worked fine), but when I uploaded to a host that did not have that add-on module installed, I would get a 500 error with very little to go on - sounds similar. It drove me crazy trying to find it.

So make sure whatever options/addons that you might have and be using locally in IIS are also installed on the host.

Similarly, make sure you understand everything that is being referenced/used in your web.config - that is likely the problem area.