When transferring ownership of an activity,
tiki-g-admin_instance_activity.php outputs the string
"caquita13" and the new owner's username to the screen.
This looks like it's debug code that got past testing.
Simply remove lines 49 and 50 from
tiki-g-admin_instance_activity.php and the problem is
solved. These are the offeding lines:
echo "caquita13"; echo $\_REQUEST\['owner'\];