Oh, hey, I have a similar issue and will try the above steps.
I flew my Mavic Pro about three weeks ago, no problems - everything fired up, connected and worked as usual.
Last night, I did everything exactly how I usually do it and it won't connect to the DJI GO 4 app. The RC connects to the Mavic, but not the app. I tried changing to the middle usb port - no success. I tried uninstalling and reinstalling the DJI GO 4 app - reinstalled fine, but still no connection.
I suspect the problem started when my phone (Pixel XL) updated to Android 9 two weeks ago.
I also checked Litchi, which just crashes immediately on start up now.
I suppose my next steps are to clear all of the DJI caches and folders and reinstall the app again.