Hi,
We were getting the below error in one of our plugins. The reason was because we were making use of HttpContext class of System.Web assembly.
For our on-premise deployment it was working fine, we were getting this issue only on registering plugin on Sandbox mode.
We can find the explanation here
http://social.microsoft.com/Forums/en-US/crmdevelopment/thread/302377cf-97bd-4fe9-b7d0-fa7f08152927/
Bye.
