Hey @Marcin-grzes,
Welcome to the Sonos Community and thank you for your post!
Sorry to hear you are getting dropouts on your left Era 300 in bluetooth stereo set-up. Great stuff, checking the software is up-to-date!
I have a few more ideas to narrow down the issue:
Have a look at this guide especially about reducing interference.
Do you experience similar drop outs if you target the players over Wifi?
Does playback from another music source via bluetooth, like your phone, also cause this issue?
Does playback on the Eras via bluetooth as single players produce the same symptom?
And lastly, when you swap the players around in the stereo pair (you’d need to reconfigure the stereo pair in the app), does the drop out disappear, move with the former left speaker or stay on the new left speaker?
Let us know how you got on!
Mark good posts by pressing the like button, and select the best answer on questions you've asked to help others find solutions.