Getting to the bottom of an application error can often take time. It's great you've already gone through some troubleshooting steps (helps those trying to assist you!). I'd recommend however pasting the exact error message zoom is giving into your question. Usually when an application errors out, there is also an error code. This can greatly help in diagnosing your issue. Here are some of my suggestions on what you could do next based on the information provided:
1. Restart your computer
I am assuming you have already done this, but half my problems on computers have been solved by restarting. I cannot tell you how many times I've told people to simply restart and everything is fixed. If you need any help on this, I found the Apple help page for this.
2. Permissions and Other Settings
You have probably already seen this page by AppleToolBox which outlines a bunch of permissions and other network settings you can change (alternatively look take a look here). In fact, it looks like you've already gone through most of it. It's worth double checking however as sometimes in these situations I've realized I forgot a step after hours of troubleshooting.
3. The Zoom Link
The issue could also lie in the zoom link you are using. Which application does it error from? All applications or just a few? Have you tried manually inputting the zoom ID into zoom and joining that way? For instance:
5551112222 is the zoom ID that you could enter into Zoom. To enter this, click the
Join button in the Zoom application. Sometimes there is also a password. This is sometimes in the link following the ID.
Finally, perhaps try different Zoom links. You could create Zoom meeting yourself and join your own meeting by pasting the link into the application you're having trouble with and then clicking it.
4. Update your macOS version
You mention you are running macOS Catalina. The current version is Big Sur. This could possibly be part of the problem. Theoretically, Zoom should support Catalina as they state Zoom works all the way back to
macOS 10.14. There is always the chance they haven't fixed something on an older version.
Best of luck figuring out your problem. I would recommend seeing if you can find the error code zoom is giving you. Post it here as that will assist people helping you. I found a Zoom page with common error codes. The post here also outlines other resources to help you on your way. As a last resort, you could also contact Zoom Support.