When editing the summary of a social format course (the cog next to the words "An open forum for chatting about anything you want to") then the link to 'modedit.php' is missing the '/course' prefix and so you get the error message:

File not found.

this is because in '/course/format/social/format.php' the moodle_url is wrong:

new moodle_url('/modedit.php', [

there is no '/course' prefix!

Also broken in M3.6: "href="modedit.php?update=".