Exactly the same issue at my place. For me, I think it started with the Firmware 4.1.8
I had the device working properly for some days before, after upgrading controlling it via HomeAssistant was gone.
On reddit someone mentions that its helpful to disable/enable “Auto locking function”, but in my case that only worked for some hours.
https://www.reddit.com/r/homeassistant/comments/1axf361/comment/ks554n9/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button
In my case I can have a working connection if I pull the battery and plug it back in for like one or two days, than it is gone again. The nuki app does work fine all the time, but thats just boring 
Edit:
Found a longer thread, this can possibly marked as duplicate: