Final Cut Pro crashes when using Magnetic Mask on any video

Deleting preferences, as you seem to have done, was a good idea, as it can solve many unexpected problems.

Since it did not, we will need to know more, beggining with: which version of FCP and macOS?

What Mac?

Try starting FCP from a new admin user account, and test. Same problem?

This is important for determining if the problem is something specific to your account.

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box. This may show issues related to third party software modifications, which often can impact FCP, and avoid a long serious of questions back and forth.

The first thing that stands out: you have the nasty "cleanmymac", which has been often called "brickmymac", and with good reason.

You should completely uninstall it, following their instructions.

You also have a lot of very old stuff, which betrays the fact that this system has been upgraded in place, and across multiple macs, for years.

It is so easy to do that, but the downside is that old stuff accumulates, and users are typically not aware of it; and something installed back in, say, 2012, is likely to break at some point, and cause system instability or crashes.

This is why it is usually recommended that, when transferring to a new mac, one should migrate ONLY the user accounts, and manually install any required applications or extensions.

For example, you have the - once great - Soundflower, but the last version 2.0b2, ceased to work a few years ago; and you even an even older version, one that did not work since maybe Catalina or even earlier.

You have other stuff, which you can see in the report, referring a 10.6 or 10.7 SDK, so about a decade and a half old.

You also have Chrome and its "keystone" background processes. These are known enemies of video software.

You have this "megaupdater" which I think is a sort of software aggregation. I'd seriously consider NOT using this. I'd only download software from either the App Store or directly from each developer site.

You have Skype, which Microsoft has discontinued.

You have Silverlight - this has not worked in years.

I think you get my point: lots of old stuff, plus things that should really never be installed.

At this point, I strongly recommend that you sanitize your system by getting it to a fresh start.

It is much EASIER now than it used to be:

1) Full backup (preferably two) 2) System Settings->General->Transfer or Reset->Erase All Content and Settings 3) Migrate your user accounts (do NOT migrate everything, otherwise you'd get back to where you started). 4) Reinstall any applications from the App Store or their developers' sites

You will be amazed at how much faster and stable your powerful Mac will be.