I received an e-mail from Dirk Jebing on my blog post on deploying TwitNotes in Sametime and how I was having deployment problems due to an exception being thrown. This is a long time ago but Dirk now found the solution which actually turned out to be a close cousin to my Cross compiling sidebar plugins for Notes 8.0.x and Notes 8.5 post.
at least for my plugin I could solve this! The two steps I did (perhaps it may help you): 1) I realized that I build my plugin with sametime 8.01 SDK. So I changed that (that did not help at all) 2) In the sametime logs my eye fell on an UnsupportedClassVersionError. So I changed the compiler compliance level (which stood on 5.0) to 1.4, as it has been with Sametime SDK 7.5.1. And now it works!
So the lesson is to always watch for the JDK level of the client you're developing for...