Whenever I open a temporary file (usually an email attachment, or download that I’ve elected to “Open with” rather than “Save”) I get an error like “/tmp/mozilla_roy0/TPS Report.odt does not exist”. I know there are other questions like this on AskLO, where the file has since been moved/deleted, but his is not the case for me because I can open these files in other programs with no trouble.
It turns out that the default way LibreOffice is installed in recent versions of Ubuntu, Kubuntu, etc., is via a “snap”, and not everyone is happy about it. Snaps have more limited access than normal programs and (the LO one at least) cannot access the real /tmp dir.
I had to completely uninstall LO, then add a LO PPA either the “fresh” for latest and greatest, or pick the stable version that you would choose on LO’s download page (6.4 at time of writing). The PPA page tells you how to add it, then you can install libreoffice from the usual graphical software installer/command line package manager.
After that, LO starts up faster and can read files normally again.