Bluetooth pairing unsuccessful

Anyone any idea why I have started to get an error message, 'Pairing Unsuccessful iMac Pro is not supported', when I try to pair my iPhone 8 Plus running 16.4 with my 2017 iMac Pro running 13.3, please? It's always worked before!

iMac Pro shows in 'Other devices' in Settings > Bluetooth; and my iPhone shows in macOS System Settings > BlueTooth (as it does in System Information > Bluetooth - but as 'Not Connected').

I've tried the usual turning BlueTooth on and off etc, and restarts.

But when I can see 'iMac Pro' in the iPhone's Bluetooth settings (it seems to come and go), tapping on it always results in this: