You've got something on your computer that is deleting temp files still in use. Typically something like a "file cleaner" or "computer speedup" program. Either find it and disable it or tell calibre to use some directory inside your home folder as a temp dir so that the program does not touch it, via the environment variable CALIBRE_TEMP_DIR as described here:
https://manual.calibre-ebook.com/cus...ment-variables