I figured it out from my end so far. I think it is related to the Ignore feature.

Maybe I'm the only person who ever used it. When I was really ill I had to put certain persons in there because they were bothering me privately to say it kindly. In fact for long time I switched off PM completely to everyone else as well.

I just cleared the Ignore boxes and the PHP warnings did not appear.

Maybe someone else could try putting me in Ignore and try to read a thread where I have posted. Like this one perhaps? See if you get all those warnings. Simple test.

I tested in PM as well and it didn't show up there today since I emptied it yesterday. It's related to "Ignore" rather than the Theme I use.

Anyway it no longer matters really.