I don’t know when and why it happened, but BTT Remote lost the license information. It still connects to my Mac Pro and my Macbook Pro where BTT is installed and licensed, but BTT Remote doesn’t recognize that there are licenses on my Macs.
macOS 10.12
iOS 12.1
BTT 2.660
BTT Remote 2.3.1
There is Better Snap Tool (App Store version) sitting on each of my Macs, perhaps they are interfering the license process with BTT Remote?
I’ve restarted everything, reinstalled BTT Remote and have even bought a fresh and new BTT license. And now I am writing here...
Andreas, don't ask why: Everything is working again. I don't know why the Pro-Features of BTT Remote were disabled on my device for some time. Perhaps a modern Christmas miracle...