Thread:Project:Support desk/Problem uploading files: Could not create directory "mwstore://local-backend/local-public/5/54"./reply (16)

You should upgrade to the latest 1.21 release to be protected against security/stability bugs that have been fixed, but that probably wouldn't make uploads work.

You should try Setting up a debug log file to see the full path it's trying to access and see if they are really present in the filesystem and writable. Set it up, upload a file or edit a page changing the size of a thumb, and disable the log again, since it degrades performance.