Getting “Cannot modify header information – headers already sent” error, but only on backend

There’s only 2 general places the problem could be:
One is the theme and two is the plugins. The actual problem is that there’s a deprecated function that’s being called.

The header already sent message is just WordPress’s way of telling you the backend broke before it could build the frontend.

I suggest you:

1. disable all your plugins;
2. install + activate a native theme like Twenty Seventeen;
3. update all your plugins
4. activate each plugin one at a time until you experience the error
5. if the error hasn’t reappeared update your theme and reactivate it.

When the error reappears you know which plugin/theme is at fault and it will narrow down your search parameters.
Otherwise if after all these steps the error has disappeared, the problem was out-of-date software.