If you are trying to import your solution from MS CRM 2011 onpremise to MS CRM 2011 online organization and you have registered your plugins in non-isolation mode you will get this error. you can easily check for the assemblies registered in isolaiton mode or non isolation mode, like below in your solution

so, connect to plugin registeration tool and update your assemblies to register them in isolaiton mode. Now you can deploy your solution to CRM online organization without any issue.

Enjoy !!