Tank underpressure

So, because my GRiSO loves me so much and likes to show her affection, she continuously throws small issues at me for me to fix. This time I believe its underpressure in the fueltank. I've noticed hissing when opening the fuelcap at fuelstations before but always assumed it was gas escaping and not getting sucked in, and there was never really any issue. Lately I've noticed when riding long distances from full to nearly empty the bike starts to stumble irregularly after the reserve light comes on, which could either be debris in the tank or the fuelpump struggling against the underpressure. My bet is the latter, especially since a couple of months back the waterdrain in the tank was also clogged at the nipple protruding from the tank. Once it even refused to start (startmotor activated but bike wouldn't as if out of gas) until I opened the fuelcap first.

So I thought I'd clean the nipple but its a bit hard to reach from the outside as there are several thick cables obscuring it. Also, unlike the water drain hole in the tank rim which is nice and obvious, there is no obvious hole for the vent, I assume its actually somewhere under the rim? Would removing the rim allow me easy access to the vent hole or is the easiest way to clear it from the outside? Since I need to do everything outside and the 'feels like' temperature is minus 18 Celcius at the moment the least time costly method would be preferable

I seem to remember that the tank breather is visible when the tank is unbolted and lifted from the seat end. certainly sounds like a blocked breather.

I fixed mine by removing the little check-valve thing on the vent line coming from the fuel tank, on the right side by the throttle body.