My FCPX has developed a sudden inability to understand clips it had previously imported

Hello - 1st post - so let's hope this makes it through moderation

Ok, so I've been using FCPX without incident for the last 2 years. I'm not doing anything complicated, but I tend to work with a bunch of different cameras putting out different resolutions and file formats... but this has been handled fine.

..until a few days ago where all of a sudden when I attempted to import some AVI files as I'd done 100's of times before, I got the message

No importable files
None of the selected files of folders can be imported.
Change the selection and try again

Now a few days prior to this (I think) I had a bit of odd behaviour where FCPX looked like I'd started it up for the first time, giving me a "what's new in Final Cut" message and it had forgotton about my libraries. I opened the libraries up and stuff seemed ok, so I didn't think about it. It may have coincided with the 2017-001 Sierra security update I put on (but I'm just sort of grasping at straws here)

So before anyone tells me that AVI isn't supported, the AVI container should be when the codec used is Motion JPEG Open DML (which this is) and it's certainly been worked fine for the past 2 years.

Obviously, I could just convert my AVI files to MOV/h.264 using Quicktime and import them like that.. .and to be honest I wouldn't have a problem with that, it's only that all those 100's of AVI clips I've already imported are in my library and now come up as blank video. So I've loads of projects with patches of nothing in them - which is a real pain, as I need to reuse a load of these at various times

Any idea what might have happened here, or suggestions on how I might try and fix it ?