I solved the issue. I'll post the solution just for others that might have
the same issue:
1. As said before I deleted indicator applet:
sudo apt remove mate-indicator-applet
At this stage you should have Telegram responding correctly but no volume
tray icon and you get an error on startup.
2a. Then I edited the current panel config file, in my case the trisquel
layout, check mate-tweak to see what you are using:
sudo nano /usr/share/mate-panel/layouts/trisquel.layout
2b. Inside the config file remove:
[Object indicatorappletcomplete]
object-type=applet
applet-iid=IndicatorAppletCompleteFactory::IndicatorAppletComplete
toplevel-id=bottom
position=20
panel-right-stick=true
locked=true
Now save and close the file.
3. In mate-tweak I switched from my current panel to another one and again
back to my panel (the edited one). This is done just to refresh the changes
(reboot didn't do it)
4. Right click on the panel-> add to panel -> Notification Area
5. Reboot, no error, volume is there and telegram reacts on left single
click.
