Apple Watch Series 2 pairing fails

My Apple Watch Series 2 on WatchOS 4.3 started draining power much faster than usual these last two days, so I ended up unpairing it from my iPhone running 11.3.1

While pairing the Apple Watch to the iPhone again, I get repeated "pairing failed" messages after initial successful pairing:

Message: Your Apple Watch is Paired

All these actions or inaction result in a failed pairing message on iPhone:

wait 20 seconds/or initiate restore from backup/or set up as new

Both devices are fully charged

Bluetooth and wifi enabled

latest OS

Apple Watch is reset to factory settings (in fact repeatedly)

iPhone hard reset

The Apple Watch is less than two years old, but I guess I'll just have to buy a series 3 now.

Apple Support won't help, as I didn't buy Apple Care for Watch.