Error when saving/updating anything

Hello,

I hope for some swift responses as this is alarming!

Anytime i hit the ‘update’ button, or anything to similar effect from the backend, i get this error:

Warning: Cannot modify header information – headers already sent by (output started at /html/wp-content/themes/blank-magazine/functions.php:25) in /html/wp-includes/pluggable.php on line 890

There was another error that read the same thing that was pointing at line 1601 in classes.php

This only started today – i have not made any changes other that to content!

The front end seems unaffected except if a short-link is used i get one of the above two errors (example: /?p=100)

Thank you for your help