Well, I had the same issue come up, I know its a year later, but hopefully someone will find it useful.
I did upgrade to 2.5 last night and still did not see an active port mirroring session in the GUI, and my probe port was still set respectively. I had a thought that "What if I create a new session with two unused ports?" What would happen? Well, I created that session, and although the OG session did not show up, the OG probe port did revert back to a normally switched port, essentially killing the mirroring session.
After that, I was able to add and delete mirrored sessions from then on out.