Headers already sent – pluggable

Hello,

I get the whitespace. However, the error is listing PLuggable.php; which is a wordpress file.

I see wp-includes functions.php:2959 in /…
includes/pluggable.php on line 876

I deleted the space after it, and the error turned into 875.

It is hinted that the error exists when a plugin has a glitch, but I cannot tell which plugin (and it isnt listed in the header error msg)