Cannot modify header error (With additional proofreading and picture uploading)

Hello to you all. My name is Joshua Silverman and I’m the Editor-in-chief of a site/podcast you likely never have heard of by the name of Constantly Calibrating — or ConCal for short.

Over the last week my site has become nearly non-functional for myself and my writers. As of this time I’m the only one able to log in — my co-admin can’t even get logged in — and even with that I have limited functionality. Attempting to visit most pages in the dashboard, posting, previewing posts, and a multitude of other actions lead to the below message popping up:

“Warning: Cannot modify header information – headers already sent by (output started at /home/content/55/9748255/html/wp-content/themes/magazine-premium/functions.php:1) in /home/content/55/9748255/html/wp-includes/pluggable.php on line 876”

This is also the same message that pops up anytime one of my writers attempts to log-in. When posting, this message will pop up as well, but it doesn’t seem to stop a post from going through as they still appear on my site.

In addition to these errors I’m also experiencing the following error whenever I attempting to use the proofreading feature plugin:

“There was a problem communicating with the Proofreading service. Please try again in one minute.”

Furthermore if I attempt to upload a picture to my site I’ll get the following message:

“Error: ‘Imagename.png’
An error occurred in the upload. Please try again later.”

This has been going on since the end of last week and sadly my skills have not allowed me to solve the problem on my own.

Any help would be appreciated.

Thanks!