Some users got this error and I found it on http://Notes.net as well but there was no solution there. Symptoms include, mails going into draft, and when doing a Help ->  About Database the error: "DUPLICATE PUBLIC name ACTION_VIEW STATUS in USE module CSENEventClass" came up, and in the console window and log.nsf the message "Addin: Agent 'wMemoOpen' error message: Error loading USE or USELSX module: Common" came up.

My solution was to replace the design of the database with an old version of the Mail template Mail50.ntf .

This seems to work ok.

If you need a copy of mail50.ntf, most client installations have it as well as the Server, or use another server's copy as long as it hasn;t replicated the corruption around.