Just had another occurence of the “Cannot modify header information – headers already sent by (output started at …)“. Luckily I was able to solve it rather quickly with the help of some fellow wordpressers! 😎
The first clue I got from Geeklog:
This is commonly known as the “whitespace problem”. (…) The problem is that many editors seem to add additional blanks (spaces) and/or empty lines at the end of a file when you edit it. (…) The fix is, obviously, to remove that whitespace from the file.
But that was not really the problem in my case; it proved to be a missing closing php tag. At the end of my wp-login.php the ?> was missing! Thanks to Krishna on the WordPress.org support forum post I managed to solve this problem!
After adding the closing tag I was able to log in again!