by Ken Task.
Sorry ... this response not a solution, per say but ...
line 27 of /config.php: call to require_once() - what's on line 27 of config.php?
Also, consider purging server caches either via Moodle Admin UI OR manually by going to moodledata/cache/ via terminal and looking for something that looks related (sessions) and removing using rm -fR 'item'.
IMHO, the 'uprade' scripts should automagically purge all caches so one can start building them up again vs the newer version of Moodle.
'spirit of sharing', Ken