See this thread: Update to 2.6.3 removed all Access Entries. Help? - #10 by uli
Did you update your firmware to 2.6.3 recently?
Forget about the Nuki support, it is a massive black hole. I sent an email on Oct. 26th - still no answer.
I called them on Nov. 5th, because my employees also lost access to our locks, it was labelled “urgent” - still nothing.