A custom extension developed may fail to deploy on the Spotfire Server, throwing an internal server error in the Administration UI, if it is not signed with a strong name key. In such cases the "PublicKeyToken=null" is the problem. 
 

The corresponding error in the Spotfire server log would be something like the following.

ERROR 2017-02-20T16:50:41,563-0800 [sp65svc1@1tsspa.lab, #107] ws.deployments.GwtDeploymentServlet: Internal Server Error a96618ad-a265-4f54-b295-aa6e87b45f68: The value 'SpotfireDeveloper.HtmlPrintToolExampleForms, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not a valid AssemblyName.java.lang.IllegalArgumentException: The value 'SpotfireDeveloper.HtmlPrintToolExampleForms, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not a valid AssemblyName.

 

Custom extensions need to be signed in Visual Studio before creating the SPK file. To sign the Custom Extension via Visual Studio, browse to the "Project Properties - Signing" - Select the checkbox to Sign the Assembly - Create a new key to sign the file or use an existing one.

Alternatively the Assembly can also be signed in the Package Builder after adding the extension. Right click the Extension - select Package Signing Options to select the required key to sign the package.

Note:  There is a SpotfireDeveloper.snk file (Visual Studio Strong Name Key File) available in the TIBCO Spotfire Developer package SDK/Examples/Extensions folder which can also be used to sign the assembly.
 

This article explains how to resolve the error "not a valid AssemblyName" when deploying a custom extension on the Tibco Spotfire Server.

Package Builder signing option
Microsoft Reference Aritcle on How to Sign an Assembly