After updaing Final Cut X I could not get it or Motion to load. The icon would just bounce in the dock and then immediately stop. Nothing.
I tried updating Catalina to 10.15.7 but no joy.

I was getting no crash report either. I set up a testing account and it failed but this time I received a crash report and saw this:

Application Specific Information:
/usr/lib/libcrypto.dylib
abort() called

Invalid dylib load. Clients should not load the unversioned 
libcrypto dylib as it does not have a stable ABI.

I then came across this site: https://dev.to/ruivieira/fixing-libcrypto-ansible-crashes-on-macos-mm2

I seem to remember installing ‘HomeBrew’ in the past and so made sense it might have been something to do with this. I followed thie instructions here and it worked.
I think I only needed to do this update:

$ brew update ; brew upgrade ; brew install openssl

So three hours later…Final Cut is working again. All this updating can be very time consuming.