When I reinstalled AHP 3.310, the strange O's did not appear, they started again on Monday, so it can't be my PRS511, it has to be something else.
It doesn't HAVE to be something else. After all: "With X10, the possibilities are
endless!"
Did you try changing the Housecode on the PRS511 to something other than "O," (preferably a code you aren't already using)?
If you do that, and the "O" codes keep happening, that should rule out the PRS511. However, if the oddball codes move to the new HouseCode, then I'd say that points back to the PRS511 again.
If you're positive that it isn't the PRS511, then what do YOU think it is?
You have already stated several times that NONE of your neighbors use X10, so the signal must be coming from inside (or immediately outside) your house somewhere. Since "receiver" modules can't transmit a code, and it is not very likely that electrical noise would be so repeatable (but it is still a possibility I suppose), I would suspect it is coming from one of the devices that you have that can transmit a code. The PRS511 falls into that category, which is why I suspect it might be the cause of your problem. It also has the ability to transmit a sequence of several house codes.