If you try to update both headphones together, it is possible that only one headphone will receive the new firmware. You can then use one headphone OR the other, but not both, as they are equipped with different firmware. In this case, simply update the second headphone with the new firmware as well.
Here are the instructions for the correct firmware update:
https://usa.1more.com/blogs...

Unfortunately I have the same problem...! Does anyone have another solution?

I would say so. After all, they still have a guarantee. I bought normal headphones from Digitec years ago and when one suddenly stopped working, I went to the shop and they gave me new ones straight away. If in doubt, send them an email. So far I have only had good experiences with the customer service.