Hi Gui,
the NL2 and NL4 Speakons are interchangeable and will mate correctly, the only difference is the NL4s can carry a four way cable often used in a bi-amp setup where two separate amps power two speakers via one 4 way cable.The second speaker is usually wired via a link socket on the cab, similar to the old peavey Hysis sub speakers.
The NL2s only carry a 2 way cable. Wired to the 1+ and 2+ connectors.
It's possible one end of your Speakons is an NL4 and is wired incorrectly with 1+ or 2+ being wired to the 1- or 2- connectors inside the plug.
I have some wired on purpose this way for a two way system link, and have to clearly mark them from the normally wired ones.
The easy option is just to check whether the connector says NL2 or NL4, or buy a new cable that uses NL2s only so no confusion.
Hope you get sorted.
MM