Unable to connect Bluetooth: WH-CH700N headset

Hello!

I use my Sony WH-CH700N headset which has a Bluetooth connection.
However, this wireless connection has never worked with smartphones (tried with iPhone XR and Samsung A3 2015). The headset is properly displayed in the list of devices to connect, but when I attempt to connect, an error message appears "Association refused by WH-CH700N."
This prevents me from using the app corresponding to the headset to change some settings.

Until recently, I was able to connect the headset via Bluetooth with my MP4 (Sony Walkman NWZ-A15). In just two hours, the connection stopped working! At 2 PM, I was listening to music wirelessly, but when I got back home at 4 PM, the connection was impossible.
I try to unpair to pair again, but that’s also impossible; the headset no longer appears in the list of devices to connect to on my player (even though it still appears on the smartphone).
The problem cannot come from the MP4. I tried to pair the player via Bluetooth with other wireless headphones, which works without any issues.

Does anyone know if this is a problem that I have not been able to resolve or if the hardware is defective?

Thanks in advance.