Faced this error very frequently during my early days with MSCRM. With all my enthusiasm, I would build a plugin and register it in my CRM. Oops! The plugin registration tool throws me an error.
Once of the reason why this error is being thrown is probably because you have forgot to sign the plugin assembly. I have seen that error usually comes up when you plugin dll is referencing other assembly and you do an ilmerge but forget to sign the merged assembly before uploading it in Plugin registration tool.
Hope this helps!