That inidicates some program on your computer is deleting temporary files that calibre is using. Typical suspects are "performance enhancers", temp file cleaners, etc. If you cannot figure out what program is the culprit, you can work around it by telling calibre to use some custom folder as a temporary directory, by setting the CALIBRE_TEMP_DIR environment variable, as described here:
https://manual.calibre-ebook.com/cus...ment-variables