Hey there Piwko,
I apologize for the late response, thank you for being patient! Were you able to fix that Bluetooth pairing issue? If not, I'd be happy to help out, as these Bluetooth connections can be finicky. First, make sure both your Chromecast and Soundcore headphones are up-to-date on any firmware updates. Then, go ahead and follow the troubleshooting in this article.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!