The backlash nut probably didn’t cause that. There are two problems I’ve had with my 2 nut lock.
One was that I had the BL nut on too tight, and that caused the Z motor to skip steps, or the driver for that motor to fail completely.
The other problem I had was that the coupler nut and backlash nut fell out completely. In my case, that was because I printed the wrong one (I should have printed the 11mm one). Once I printed the 11mm one, I had to push the nut in with some pliers, even before I tightened the screw that pulls it shut.
Another person here (was it Barry?) added some tape around the coupler nut to make it a bit thicker, in case you already printed the 11mm one, or you don’t want to replace the nut lock.