When used with an Apple Watch, I get the error "Connection failed" and "Melomania Disconnected"

Try this solution

Manually pair the secondary earbud

1
Go to the Apple Watch Bluetooth settings and check which earbud is connected to the Apple Watch

2
Press the button on the earbud not connected to the Apple Watch for 3 seconds to enter pairing mode. You should hear the voice prompt "Waiting to pair".

3
Pair with the earbud in pairing mode from the Apple Watch Bluetooth settings