I have a project that is almost entirely still images with transform effects. I upgraded to the latest Appimage today and when I reopened the project, I first get a dialog with errors that includes: "sans serif will be replaced with Dejavu sans"
If I confirm that message, I then see another message with this error for every image/clip in the bin

invalid clip producer 32 found on track playlist2 at nn (where nn is some number)

All the clip thumbnails are still shown in the project bin and the files are unmoved. If I locate the clip, the file manager finds it correctly. Selecting any of the images in the bin shows 'invalid' in the preview. The clip properties shows them as text clips with Dejavu sans as the font and 'INVALID' as the text. It looks as though all my image clips have been converted to text clips.

How can I recover this?

The issue is resolved. Traced back to an improperly configured NFS4 server. I don't entirely understand the issue but resolving the NFS issue also 'cured' my broken project.
I know this was not a simple access issue as kdenlive could see and access the files and I could access them from other applications on the same machine.