headers already sent message means that something — a plugin or a theme, usually — is sending output before it’s meant to. Often this is due to whitespace appearing in a file after the closing
?> tag, which can make it hard to find the culprit.
The fact that
pluggable.php is the apparent culprit tells me that it’s most likely a plugin that’s replacing some of WordPress’s pluggable functionality. Often this is related to user management, though there are other pluggable functions.
The usual advice in this situation is to turn off all your plugins and switch to a default theme. If the problem goes away, then the culprit is a plugin or the theme you were using. Turn them back on one by one until the problem recurs. Now you’ve found the culprit. You will then know who you need to reach out to for support.
Pluggable Functions — it’s possible, too, that there’s a plugin attempting to do HTTP->HTTPS redirection, and that might be causing you grief.