There is a site setting that controls the length of messages sent from the Participant Center or tell-a-friend emails, and if you attempt to send a message that is longer than that value it will produce this error message. To fix this an Advanced Site Admin may be able to set the value for the SDP "TAF_LENGTH_LIMIT" to either a larger value, or blank (an unlimited size). If you cannot see this site option under Setup > Site Options or do not understand how this option works, please contact support and mention this solution ID to have this changed for you. 

Note: We are currently working on making this error message more user-friendly.