Problem with the ical feed of the calendar
As discussed in https://lab.civicrm.org/dev/wordpress/issues/25 and using a different grep since the grep linked to in the comments above doesn't seem to work, there were two theme files with a BOM.

This grep

find . -type f -printf "%p " -a -exec od -x -N 4 \{\} \; | grep "bbef 3cbf"

located

wp-content/themes/ari/functions.php
wp-content/themes/ari/header.php