Logic Pro X 10.5.1 Now stuck on loading screen 'Updating Info About Audio Unit Plugins'

You need to access it from the hidden Library folder:

1. Go to the Finder (or desktop).
2. Hold the Option key on your keyboard, and click the Go menu at the top of the screen.
3. With the Go menu open, you'll notice that pressing and releasing Option will display or hide the Library choice in this menu.
4. Select Library from the Go menu (while holding down Option) to access the hidden folder.

In there select Caches/AudioUnitCache

In my case there were two cache files in there which I had to trash (both).

Upon restarting Logic it rescanned for plugins and recreated the caches.

Once again, this is what I did with Apple support, but it might not be the correct course of action for you.

I have just been talking to Apple about this exact problem, and in my case it was resolved by trashing the com.apple.audiounits.cache. When I restarted Logic it rescanned all of my plugins and told me which ones were incompatible. In my case it was a bunch of legacy Steinberg plugins that I never used anyway so I just deleted the lot. By the way, this solution might not work for you and my recommendation would be for you to call Apple if you are not comfortable with tinkering. I am just telling you what solved my problem and take no responsibility for what you might decide to do.

Same problem here and I am livid. I am right in the middle of a project and this happens. Apple please resend with a solution.

Same problem here too. Working fine last night. Now today this, right in the middle of a deadline! Sort it out Apple.

It doesn't work on my mac, i have the same trouble.