CRM 2011 Error "Assembly must be registered in isolation" when importing new solution containing a plug-in.

Greetings.

For the first time, instead of deploying the plug-ins on disk,  I tried to include the plug-ins into the solution; so I created 4 Web Resources "Plug-in Assemblies".
The "Isolation Mode" of all the plug-ins is "NONE".

Everything worked correctly on the Development server; the issue occurred when I deployed the solution on the Test server.

Trying to import the solution in Test Server, I get the error: "Assembly must be registered in isolation".

I thought that after registering the assemblies and including them in the solution that the solution could be imported on any other CRM deployment and that there was nothing else to do about the plug-ins.

Am I missing something? Could anyone help?

Thanks,

Frank