we had a similar issue, in our case it was the .Net Version we use for the plugins and the version installed on the server, plugins using version 4.5 and server only had version 4.0, or similar versions, after updating the servers with the correct .Net framework the Sandbox Service was running without any issues
also you can follow the conversation on the following post: http://social.microsoft.com/Forums/en-US/e1a33311-e908-433e-b551-3c751e5c317b/crm-2011-online-plugin-communicationobjectfaultedexception
abe saldana