Quoting golinux@??? (golinux@???):

>> What is logged in /var/log/auth.log when you run pkexec?
>
> Not readable. I tried a few iso codes from the very long list but
> none could read it. UI was root at the time.

In a terminal, 'less /var/log/auth.log' (as root) ought to always work.
auth.log is outputted by the rsyslog daemon, as you can see in
/etc/syslog.conf -- and in my long experience has always been pure
ASCII.

> "The document was not UTF-8 valid
> Invalid byte sequence in conversion input."

How very odd. But do try the good old 'less' utility. Even though
there _shouldn't_ be peculiar characters in /var/log/auth.log, if there
really is such content, 'less' won't be overly impaired by same.