Hi Jaime & @Juergen
So I installed the 3.8.2. There are less problems (running UI Express and U6+ as WLAN AP). This ONLY works when sending pings every 30 secs (and killing the Nuki-battery). BUT…
Unfortunately that does not mean that there is anything stable, not Nuki Web not the App in the same WLAN, even in the green times 
NOTHING stable. Sometimes full stable connections for a few minutes.
It is so hard to understand what causes this. @Jaime_Creixems: Does the Nuki Web or the App in the same private WLAN works without a problem?
I changed the UI AP to a Fritz WLAN Repeater in that UNIFI Network, just to see if the AP causes some issues (that worked before on a fritz.box invironment 100% stable). ==> Don’t work. So the Unifi AP is not the bad guy in the drama. It seems that somehow the connection to internet has a problem and then the nuki lock is sulky don’t come back with the tunnel.
I disabled important parts of the Unify Firewall, nothing helped:
Nuki 3 Pro even worse:
Final I will try with a Unifi Dream Router instead of Express. And then, if it don’t work again, I burn all that unifi down 
Someone an additional idea?