Many of us here use FCP professionally every day and have not experienced the issue you're encountering. Have you tried resetting FCP prefs? Hold Option and Command while launching FCP and choose Delete Settings. It will open the default Untitled library. Re-open the library you were working on, set the Settings back to the way you like them (disable Background rendering, etc.) and see if the issue has been resolved.
Does the issue occur with every library or just this one? Create a new library and import the mp4 that complains about a missing plug-in in the other library.
Another trouble-shooting technique is to create another admin user, then login as that user and open the problem library to see if there is conflicting software in your primary login.
If these don't solve the issue, post the output of an https://etrecheck.com report using the "Additional text" button in the reply.
Regarding other NLEs, the free version of Davinci Resolve is popular, though the learning curve is steeper. Premiere is also popular but is a subscription. If you don't need a full-featured NLE, there's also the free iMovie.