While registering a plugin assembly through plugin registration tool we got the below error
The reason being we were using MSBuild.ILMerge.Task Nuget package and there were few assemblies being referenced in the plugin project that had copy local set as true.
So eventually the single plugin assembly that was getting build had these assemblies also being merged in it, which led to the above error.
Setting Copy Local as False for those assemblies fixed the issue.
Hope it helps.