Private Message Indicator but no new Private Messages
You can try the "mark all as read" option.
If that doesn't work, log out of RPoL, clear your browser's RPoL cookies, then log back in.
You might have to read and/or mark as read some messages again, but usually false unread indicators are due to a browser cookie issue as that's how RPoL tracks what you've read and what you haven't.
I think there's a rare instance with PMs where if you delete a new PM on your end without reading it, the unread indicator can get stuck, but it doesn't sound like that's your issue.