Was getting this Error in my Eclipse IDE after moving to Notes 8.5.1. Class NotesException derives from the CORBA package’s UserException class, but UserException does not appear in Notes.jar.
In 8.0.x and 8.5.0, the CORBA package was included in rt.jar resident in Notes\jvm\lib\ext. 8.5.1 shuffles around all the jars, and now the CORBA package is in ibmallorb.jar, found in Notes\jvm\lib. So update your project’s Java Build Path > Libraries dialog, via the Add External Jars button.
It took awhile to figure this out. Hopefully this post will save others some time.