Various "Cannot modify header information – headers already sent by" Errors

Hello,

After updating for WordPress version 3.8, I’m getting a lot of “Cannot modify header information – headers already sent by” errors in different pages.

For example in the media tab:

Warning: Cannot modify header information – headers already sent by (output started at /home1/mgph/public_html/wp-content/themes/Designrshub/functions.php:1) in /home1/mgph/public_html/wp-includes/option.php on line 563

Warning: Cannot modify header information – headers already sent by (output started at /home1/mgph/public_html/wp-content/themes/Designrshub/functions.php:1) in /home1/mgph/public_html/wp-includes/option.php on line 564

There are also same errors in the login page but different lines in the option.php and pluggable.php

Though I’ve checked the function.php and made no changes since then (before and after the error occurred), I can’t seem find the error (or maybe because I’m not a web developer).

Any help pls?

Note: I’ve checked related threads and did the advice but that didn’t fix the error. Thanks.