Important note: The 2.6.1 build does offer a new BLE service (Device information service), which is not used by the Nuki Apps, but seems to have disturbed iOS in connecting to the Smart Lock. Therefore iPhones which have been connected to the same Smart Lock in the past, could not connect via Bluetooth anymore until the Bluetooth cache of the OS was flushed (which happens when you switch Bluetooth off/on or reboot the phone). As a consequence commands that are never sent over a bridge but always directly via Bluetooth did fail (e.g. Auto Unlock, Calibration, Door sensor configuration). This is the root cause of most of the problems mentioned in this thread.
A new binary with a workaround has been uploaded on Monday 7.10., 1pm. Everybody who did update after this is not affected by any problems. The ones who updated before will get the old status restored with an upcoming 2.6.2 update, but will - unfortunately - have to restart their iPhones (or switch Bluetooth off/on) again …