Yup, it was.
https://vborg.vbsupport.ru/showthread.php?t=109096
I was able to debug it down to a plugin...it's not even one that shares a hook...
In that product, it was the plugin that goes to hook location threadfpdata_start.
I ended up turning that hook off, and everything was fine. Turned the hook off and on a few times...and then it got happy and now works. No clue in this world why, I didn't change any of the code at all.