Cannot change any Bluetooth settings On my MacBook Pro 2018 I can't change any Bluetooth related settings anymore as long as no external dongle is connected.

Examples: If I turn it off, next start it is turned on again. If I remove a device, next time it connects automatically again. I can't see any new devices nor add them. 

It just seems like there is a Bluetooth config somewhere which has been set to read-only.

I've already tried everything I could find via Google: Reset the Bluetooth module via the debug menu, remove all Bluetooth-related .plist files, even completely wipe and reinstall macOS, but nothing did help. I tried Mojave and High Sierra.

I already tried to get some Debugging info but "Bluetooth Explorer" from the Additional Tools for Xcode by Apple does not start at all, so I can not access any information.

Edit regarding the external dongle: When an external BT dongle is connected, everything works just fine. But obviously, I can only change settings and devices of that dongle. When I plug it out again, the problems reappear.
Try resetting the Bluetooth module.

⇧Shift⌥Option mouse click on the the Bluetooth menu bar item > Debug > Reset the Bluetooth Module.