Find missing/moved video clip Kdenlive
The only way I've found to fix this is to save my project, close KDEnlive, then relaunch and open the project. KDEnlive will pop up a box with all the missing clips and give you the option to search recursively to find them.
Once it finds each clips' new folder, click "OK".
Be sure to save your project again after KDEnlive has made the changes.
(aside: would be nice if KDEnlive had an option to perform this recursive search when right-clicking a clip that has gone "missing"... maybe someday)