I've seen it effect the FEL when the 3PH lever is in the "UP" position and the 3PH goes into full relief.......the FEL will not go down like normal cuz it needs to return through the 3PH.....
That being said, QC's not connected will do the same as the OP'r problem...
My thoughts are that if the FEL is using all or most of the fluid, the 3pt will not get any or very little. If the 3pt is in the up position, any fluid the 3pt gets is bypassed to the reservoir or relieved. Any fluid used by the FEL valve with PB, is passed out the OUT port of the FEL valve to tank.
If fluid can not leave the 3pt valve, fast enough, back pressure could build up, and reflect back to the FEL valve.
I am not sure what the effect would be if the FEL valve only has an OUT/TANK port feeding the 3pt when in the up position. It would be how much fluid the bypass mode will let out to the tank.
If the QD's are blocked/unplugged, the relief should go off. The QD's could be partially blocked and slow things down.
I sure wished everyone would use a hyd gage. I believe it would help answer/solve about 50 % of the problems.
If you know what the pressure is supposed to be, and if it is not there, then you have a starting point for trouble shooting.