A November update to McAfee's VirusScan Enterprise 8.5i is the culprit, both companies said in advisories posted to their Web sites.
"While working in Lotus Notes mail for some period of time, typically an hour or two, upon opening or deleting a message, the following error appears: 'You are not authorized to perform that operation,'" reads the IBM advisory.
Users must restart Notes to regain control of the program, but the problem will resurface in another hour or two, adds the McAfee warning.
Although McAfee is investigating the issue, the only solution now available is to disable virus scanning of Notes mailboxes and databases. The security vendor included instructions in its online warning for turning off such scanning.
Notes Client R6 and R7 are affected by the VirusScan Enterprise bug.