I splash my hot wort into my no chill both at flame out and after a 15 min rest with no disernable cardboard flavours after fermentation.
Use 2 thermomenters you might be mashing to high.
I have whirlpooled/not whirlpooled so maybe skip this step as previously mentioned.
I have even fermented kettle trub with out real bad tastes coming through.
Wort should be VERY sweet at room temp.
Aerate by pouring from no chill instead of aeration stone.
Leave out finnings. Change yeast. change hops.
Anyways all of these have been mentioned above,
No onto BIAB specific problems mostly hypothetical, some even outragous!
Did pot have a plastic layer that should be peeled of the inside base of your pot?
was the swiss voile from the outside of a roll that has been sitting in a container for years and picked up all that dust stink?
how do you wash the bag?
what cleaner/sanitiser are you using?
are you adding salt or anything but tap water malt hops yeast?
is your water coming from cold tap or hot tap in the house?
is water coming from a plastic hose?
is thermometer melting?
does the lid have plastic that is sweating at boil into your wort?
is your grain or hops living in non food grade plastic?
is the nochill food grade ? not fuel grade?
are fermenters food grade/clean etc?
I know how the above sounds but I hope theres a little help in the above post somewhere