The apparmor profile should be set so that the various filters like gzip can write to /var/cache/man/**. This would stop the error from appearing when logged in as root or /usr/bin/man configured as setuid.
David
This bug appears to be caused by the installed apparmor profile for
man-db in buster. If you are the root user and/or /usr/bin/man has been
configured as setuid and there are cat? directories in /var/cache/man,
then man will try to write a cat page to those directories but will be
prevented by apparmor.
- Bug#926450: spurious error "gzip: stdout: Permission de... David Sanders
