Apple Watch Ultra (first gen) won't pair with plantronics backbeat fit 3200

Hello, I am having problems connecting my sports earphones (Plantronics backbeat fit 3200) to my Apple Watch Ultra (first gen).

The Ultra finds the earbuds in the Bluetooth list, but cannot pair them. I get the message;

make sure the headphones are on and within reach.

And then it doesn't do anything anymore and is therefore not connected.

The earbuds work perfectly on my iPhone 15 pro and also on my Garmin Venu. But now that I have switched from Garmin to the Ultra, I would like the earbuds to also work.

Anyone have an idea?