Importing a solution in Dynamics CRM usually goes off without a hitch. There are times though when the error reported is not easy to decipher. The error below is an example of one of them - a plug-in assembly error. The error was displayed as the solution failed to import.
Initial attempts to ensure that the plugin assembly and steps where included in the solution were performed but re-attempts at importing continue to fail.
One of the things to look our for during continuous development is code clean up or changes. This may be a renaming of a class name or the removal of a class all together from the assembly.
With the error above it was tough to determine exactly what about the plugin was different. In this particular case a class was removed from the plugin assembly, so when importing into the new environment CRM had an issue updating the assembly and reported the above error.
One way to determine if anything has changed is to compare the classes in the assembly using the plugin registration tool. It is probably the fastest way to identify what is registered in the source system and will give you the ability to connect to the destination system to correct the problem.
Note: In the latest SDK to Plugin Registration Tool is located under SDKToolsPluginRegistration
In this case the fix was to unregister the removed class of the assembly from the destination. Once complete a re-attempt to import the CRM solution was done and then imported successfully.