MacBook doesn't find any Bluetooth device After running sudo nvram bluetoothHostControllerSwitchBehavior=always because I wanted macOS to use only external Bluetooth dongles instead of the internal one I cannot find any Bluetooth devices anymore when the external dongle is not connected. I already ran 
sudo nvram -d bluetoothHostControllerSwitchBehavior
 to reset the setting, restarted macOS several times, reset the NVRAM, reset the Bluetooth module, etc..

Is there anything I missed after using these Bluetooth devices with my external dongle? When now plugging in the external dongle again, it finds all the devices properly.

I tried to search for the devices with my iPad (Magic Keyboard 2), and it instantly recognizes it.

Edit: I just erased my macOS installation and reinstalled High Sierra, but the problem persists.

Edit2:

The system report is showing me that seemingly there are devices connected (when the internal device is active) although I removed all devices via the menu bar option. Is there maybe a correlation? Is there an option to manually remove the bluetooth connections?

Devices (Paired, Configured, etc.):

24-1b-7a-21-aa-da:
Address:    24-1B-7A-21-AA-DA
Major Type: Miscellaneous
Minor Type: Unknown
Services:   
Paired: No
Configured: Yes
Connected:  No
Class of Device:    0x00 0x00 0x0000

ac-e4-b5-c9-98-6b:
Address:    AC-E4-B5-C9-98-6B
Major Type: Miscellaneous
Minor Type: Unknown
Services:   
Paired: No
Configured: Yes
Connected:  No
Class of Device:    0x00 0x00 0x0000

78-fd-94-5e-f6-a6:
Address:    78-FD-94-5E-F6-A6
Major Type: Miscellaneous
Minor Type: Unknown
Services:   
Paired: No
Configured: Yes
Connected:  No
Class of Device:    0x00 0x00 0x0000

dc-9b-9c-ca-a4-e0:
Address:    DC-9B-9C-CA-A4-E0
Major Type: Miscellaneous
Minor Type: Unknown
Services:   
Paired: No
Configured: Yes
Connected:  No
Class of Device:    0x00 0x00 0x0000
I had the same problem, after inserting an external bluetooth dongle my Macbook pro 2018 internal bluetooth control has gone, tried everything, resetting SMC, NVRAM, installing a bran new Mojave on an USB disk
I opened a ticket to Apple and after a morning of retries they concluded my internal controller hardware is broken.

I suggest you to run "Bluetooth Explorer" (I was able but "Bluetooth Explorer" does not detect my internal controller..)