Envrironment:
OS Ubuntu 20.04
browser Opera 76.0.4017.123
I always use Bitwarden in mobile, Ubuntu, Windows and as a Firefox extension but, under Ubuntu, Opera Bitwarden extension does not really open (after a long time, no action).
I removed and re installed the extension without any change.
Any idea?
Well, just one idea: your Opera version seems heavily outdated. Current version is 108. My first step would be: update Opera and see if this changes something. 
I updated my Opera installation to 76.0.4017.123 (last version now), but Bitwarden extension does not open (it seems opening but after some minutes is still running to open).
What can I do?
It seems to me that this was no real update - your version now (76.0.4017.123) is the same version as in your first post (76.0.4017.123).
I’m no Opera user - maybe the newer version is called “Opera One” now? I stumbled on this, as I searched for this: Opera 108.0.5067.24 stable update - Blog | Opera Desktop This seems to be the latest version of Opera (One). Can you download and install this on your Ubuntu?
But maybe someone else can chime in who knows Ubuntu and Opera better than me?