So I pulled up my control panel page listing all my subscribed threads. That is a tab I have previously opened, but I hit refresh. The list of threads displayed showed a number of old threads that haven’t been active for months - the last post in one was 2013, others seemed older. I was thinking the board was hit by spammers that had been disappeared, but then I noticed it wasn’t just one or two threads, but none of the current threads I have been active in were showing up.
I refreshed the page again, and now the behavior has reverted to normal.
Um, What the hell happened? 
At least it’s working now. fingers crossed