Trying to add a new WLAN - error code Eule/Owl

Since the box has to be up and running next Monday, I haven’t implemented MacVlan yet. Instead, I flashed the box with the new IP of the TC (and the current network data, just to be on the safe side).
TC is online and usable, Boxine is online, the box is also displayed as online in the Fritzbox and TC (but the display for the status/-time is delayed?) – but I still get the code word “Eule/Owl” when I try to refresh or load a new Tonie…
What have I missed now? Toniebox and TC now have permanently assigned IPs in the Fritzbox, which I will then transfer to MacVlan later or reset as soon as I set it up.

In addition, the box no longer starts when the battery is connected? Without the battery, it starts via the charging station, but with the battery, nothing happens. During flashing, it still worked normally with the battery, but then it wouldn’t turn on anymore…
Actually, the battery was fully charged before flashing; I took the box directly from the charging station, where it had been for over 24 hours. So the charging capacity can’t really have been so low during the procedure that it no longer starts up—or can it?
All cable connections etc. look fine, and I didn’t accidentally pull on them or anything like that while flashing or installing/removing them.

Edit:
No entry for failures in the TC/Portainer logs…