SUMMARY
Autosave does not work if project is placed in directory with long path to that directory, especially if it contains national alphabet characters.

STEPS TO REPRODUCE
1. Create directory '/tmp/Поддержка национальных алфавитов? Слишком сложно!'
2. Save project there as "project.kdenlive"
3. Fiddle with project a bit to let Kdenlive notice that there are unsaved changes that might be lost

OBSERVED RESULT
You will every minute or so get error message "Cannot create autosave file $HOME/.local/share/stalefiles/kdenlive/VERYLONGSTRING"

EXPECTED RESULT
Well, user should not be pestered with error messages.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: 5.16.5
KDE Frameworks Version: 5.61.0
Qt Version: 5.13.1

ADDITIONAL INFORMATION
According to my tests autosave file is named by pattern "$hash" + "$url_encoded_path_to_project_dir" + "$a_bit_more_chars". These scheme easily produces really long filenames! Because filenames under GNU/Linux has to fit in 255 bytes, scheme leads to inability to save file and error messages.

This is especially common problem if one uses national characters in directory path. For instance russian "Я" becomes "%D0%AF" - 6 bytes, 3x times more than it takes in original directory path, where project resides.