@yamahito thanks for reporting your trouble with the Online Player for Linux.
I have just tested the Online Player for Linux downloaded from https://syrinscape.com/online/ in an Ubuntu 20.04.03 Desktop VM freshly installed in VMware Fusion on macOS.
I was able to extract the archive to my desktop and run the player from there. I did have one issue where once linked, it would not allow me to
Activate This Device. I quit and re-launched the app and then I was able to activate the device.
I am able to control it via the master interface and sound, background art, and the audio visualisation is working.
You might want to take a look at the Unity player log for more clues. On Linux it can be found at:
~/.config/unity3d/Syrinscape/Syrinscape Online Player/Player.log
On macOS it is:
~/Library/Logs/Syrinscape/Syrinscape Online Player/Player.log
On Windows it is:
%USERPROFILE%\AppData\LocalLow\Syrinscape\Syrinscape Online Player\Player.log
I believe this file will be rotated on every launch of the player. You can DM me the content of this file and I’ll take a look.