Yesterday, I discovered I was unable to login to the admin dashboard of my MU site (or to the dashboard of any of the subsites) because of this error: Warning: Cannot modify header information - headers already sent by (output started at /home/mywinterthur/mywinterthur.org/wp-login.php:62) in /home/mywinterthur/mywinterthur.org/wp-includes/pluggable.php on line 1179
No themes or plugins have been installed since last successfully logging in for weeks. Nor have any PHP files been edited in a very long time. Essentially, saying there's no obvious connection between any admin work and this error.
Most of my sites went through an automated update within the last couple days, including this one, I believe, though to be sure, I'd have to go find the email stating that. This is my first attempt at logging into the MU admin since getting that flurry of emails.
Since I am unable to get into the admin dashboard, the vast majority of "fixes" that I've found online are not applicable since the first thing they say is to login to the dashboard. A small sample of the others call for reinstalling Wordpress or other fairly extreme measures that don't sound particularly applicable to fixing this issue.
Digging into pluggable.php, the error seems to be occurring in the wp_redirect function when it actually tries to do a redirect: header("Location: $location", true, $status).
I have read WPMU's article, "Fixing the “Cannot Modify Header” Error in WordPress" but the remedies suggested in there, I don't think it apply in this case.
I am hoping you have seen something similar and have a suggestion on how to proceed.