I think it knows what it was. I did not use a separate floor and wall segment, i used a "room" segment. So i thought, look at the whole picture, what could be wrom with that? It's only with the light effect.
Then i looked at the whole room and slowly towards the fireplace. It doesn't stand out so much because of the naturally seaming ability of a carpet wood floor, but it looked different. That could only mean it is rotated in a way that doesn't match the rest of the room, so i sellected it, pressed R.., R... BINGO !
Tested again, no concrete in my living room

Thank you both for you're help, by focusing on the segment, and not the added light i probably saved tons of time figuring it out!
Cheers!
Only problem left is figuring out why lightcandle.fpi doesn't flicker like a candle, it seems a normal light.
*EDIT* ok a quick google search brought me to the FPSC site:
"Added LIGHTCANDLE.FPI to simulate the luminance of fire and candle light in a room (for
dynamic lights only)"
ps if any future reader is interested; the fire placed was made by the following decals (entities, generic, Benge... emission):
2x Torchflame
2x Hotflame
Orange-reddish light with "lightcandle" as main script.
Basically 2 torch in the back, spread, then 2 hotflame, bit at front, a bit closer towards each other.
Keep experimenting with the layout for the best result.