Thanks. Tried the first two without joy. Fedora doesn't have a /var/cache/fontconfig. It's been moved to /usr/lib/fontconfig/cache. Is this relevant to the problem?
Was reluctant to try sudo rm /usr/lib/fontconfig/cache. Tried temporary mv to hide it away. No joy.
I tried the obvious fix of creating a symlink
ls -l /var/cache/fontconfig
lrwxrwxrwx. 1 root root 25 Aug 30 11:12 /var/cache/fontconfig -> /usr/lib/fontconfig/cache
but the problem persists.
Anyway, happily using the appimage.