I kept getting this error after upgrading my Lotus Notes to 8.5. Here's how I resolved it - thanks to a search on Google.
There are two options for resolving this issue.
Option A: 1. Open the mail file in Lotus® Domino® Designer. 2. Select Shared Code. 3. Select Script Libraries. 4. Select CSEventNotes and delete it. 5. Replace the design on the mail file.
Option B: 1. Open the user's mail file in Domino Designer. 2. Select Tools. 3. Select Recompile All LotusScript.