I'm facing an old error in the new UI (with dots grid on the background): the source button is gone in the left pane.
THIS IS CRITICAL!
-When I first noticed this, I tried to enable some features in Settings. I didn't find all settings mentioned in advice. This didn't help.
-The I switched back in the Authoring versions to the oldest one (without dots grid) and it helped.
I only prayed that then oldest Authoring version stays available.
But today it wasnät anymore available.
So I'm facing this problem and cannot continue work!
What a stupid malfunction from MS!
Please don't suggest Settings that cannot be found.