swagger-ui returns 500 after deployment

When debugging I was using the debug config (Which I had generated XmlComments for: Properties -> build tab -> Output -> XML Documentation File)

I had not done this for my release configuration (duh...) - now everything works

thank you @VisualBean.

As it was not so obvious for me .... how to... a simple image.

In Project > Your Project properties > Build Tab

Swashbuckle is hiding the real error message due to your customErrors setting in web.config. If you set customErrors to off you should get a better error message.

<system.web>
    <customErrors mode="Off"/>
</system.web>

As stated in the accepted answer you have to make sure the XML documentation file output is in bin and not bin\Debug or bin\Release (verify this for all build configurations).

I still got the 500 response because I use multiple XML documentation files. In my SwaggerConfig implementation I include XML documentation files from two projects (the WebApi project itself and a class library that is referenced by the WebApi project):

c.IncludeXmlComments(string.Format(@"{0}\bin\MyWebApiProject.xml", System.AppDomain.CurrentDomain.BaseDirectory));
c.IncludeXmlComments(string.Format(@"{0}\bin\ReferencedProject.xml", System.AppDomain.CurrentDomain.BaseDirectory));

The XML documentation file of the WebApi project was published correctly to the bin folder of the site, however the XML documentation file of the referenced project was not (even though it appears in the bin folder of the compiled project).

So you need to modify the WebApi project file (.csproj) in a text editor and add the following sections at the bottom (replace ReferencedProject):

<PropertyGroup>
  <CopyAllFilesToSingleFolderForPackageDependsOn>
    CustomCollectFiles;
    $(CopyAllFilesToSingleFolderForPackageDependsOn);
  </CopyAllFilesToSingleFolderForPackageDependsOn>
  <CopyAllFilesToSingleFolderForMsdeployDependsOn>
    CustomCollectFiles;
    $(CopyAllFilesToSingleFolderForMsdeployDependsOn);
  </CopyAllFilesToSingleFolderForMsdeployDependsOn>
</PropertyGroup>
<Target Name="CustomCollectFiles">
  <ItemGroup>
    <_CustomFiles Include="..\ReferencedProject\bin\ReferencedProject.xml" />
    <FilesForPackagingFromProject Include="%(_CustomFiles.Identity)">
      <DestinationRelativePath>bin\%(Filename)%(Extension)</DestinationRelativePath>
    </FilesForPackagingFromProject>
  </ItemGroup>
</Target>

See How do you include additional files using VS2010 web deployment packages? for a full explanation.