How to debug the user login process

After the Catalina update I can't login with my main user any more. Mostly all I get is the turning wheel but once I got Welcome message notification and clicking that started Safari. And from the Apple menu I was able the system settings and the app store.

I also have a few secondary users and I can login to those just fine. Which is different to the very many reports of user who where logged out after the Catalina update. Some where the user was left with deleting the hard drive to solve the problem.

Since I still have one working admin user I hope that I'm spared that fate. It seemed that the update itself was successful and the OS itself is still in working order.

I can also login to the damaged user via su.

What I want to know is: Is there a way to debug the login process to see where the login hangs?

Cleaning out the ~/Library/LaunchAgents directory worked for me.

See https://discussions.apple.com/thread/251132653

I'll find out later which of the auto start applications fail to load.