I don't know whether this is something Zoom is somehow doing 'wrong' in its interaction with Ubuntu's magic link sniffing system, but:
I launch Zoom, and try to authenticate with a Google account.
After a few seconds I see two notifications from Ubuntu saying 'accounts.google.com contacted, waiting for reply' and moments later Google opens a window saying 400. That's all we know.
If I change Settings > Default Applications > Web Browser from 'in an application based on the contents of the url' to 'in the following application [Google Chrome]' and hit 'apply'
NOW Google asks me to choose which account to use, and selecting the right one gets me in.
I don't know what Ubuntu's magic selector thing is doing, but my guess is that the problem is there, and not actually with Zoom: This same magic messes up link handling from Slack too.
I had a similar issue, but then Zoom on Flatpak changed the way it processes browser-based sign-in flows to direct users to their desktop browser. Then, it started working for me.
I don't know whether this is something Zoom is somehow doing 'wrong' in its interaction with Ubuntu's magic link sniffing system, but:
I don't know what Ubuntu's magic selector thing is doing, but my guess is that the problem is there, and not actually with Zoom: This same magic messes up link handling from Slack too.