fcpx 10.5.4 update problem (OS big sur 11.5.2)

Did you update the OS as well? From what did you update?

Trash your preferences. Hold Opt-Cmd as you double click the library you want to open to launch the application. A dialog appears asking you if you want to trash your preferences.

Please post an EtreCheck report  https://etrecheck.com  giving the application full disk access in System  Preferences. When it finishes the scan, click Share Report and choose "Copy Report”. Use the Additional Text button to paste the report into the reply.

There’s something wrong with the system. Try making a new admin user and testing the application there. Tell us some things in detail of you’re doing and what goes wrong in what media in what project properties. We need to be able to reproduce this or the system will have to be clean installed.

User Launch Agents:
  [Loaded] com.google.keystone.agent.plist (Google, Inc. - installed 2021-08-23)
  [Loaded] com.google.keystone.xpcservice.plist (Google, Inc. - installed 2021-08-23)

It looks like you're using Chrome or a Chrome based browser.

This will explain why and how to remove the problem files.

https://chromeisbad.com

You'll probably have too reinstall FCP from the App Store afterwards. There might be other things, but that's where you should start.

Oh yes. Removing Chrome is only part of the problem. The application's been damaged and has to be replaced.

The problem is not new; it just effects some systems more than others and quite suddenly. It probably depends on what the browser has been doing.

I don’t know. Try it and see how I goes. I only use Safari and Firefox.