Then it's reasonable to expect that the black in the switch controlling the hall light is its switched line and the red of the same cable is the feed that should be connected to both commons. Then the red and black of the other cable would be the two strappers and connected to L1 & L2 of the 2-way. I expect that this is what you have now?
If now both switches have to be in a particular position for the light to come on, and either switch can turn it off, this implies one strapper is broken or one switch is not making contact in one position. I.e. there is a circuit L1-L1 but not L2-L2, or vice versa. However, there was a time at which the 1g could not turn the light off. so if the connections there have not been changed, both strappers are probably continuous. Are you sure red and black of the same cable at the 2g are securely connected to L1 and L2 of the 2-way?
My next test would nonetheless be to isolate the circuit and test in turn the continuity of the two strappers, red and yellow at the single switch to red and black at the 2g. There are tests one could do live but good practice suggests working dead if possible.