A user in our environment is attempting to modify the subject line of emails along with adding categories to a shared mailbox. Recently we've created a new mailbox and when the user is attempting to make modifications "This function cannot be performed because the message has been changed" pops up along with "This item cannot be saved because it was changed by another user or in another window. Do you want to make a copy?"
On other forums I've found these suggestions and tried them:
-Cached Exchange mode is "Off"
-When a reply modifies, don't move the original
Previously with the same workflow this user was using the old mailbox wasn't having this issue. Now in the new mailbox this pops up roughly 6 times a day sporadically when they are working. If they wait a bit and then try again usually Outlook's error message will go away and not present any further issues.