"Warning: Cannot modify header information" when logging in or out

Hi, one of my blogs on Multisite install keeps coming up with an error whenever it needs to log in or out.

Here’s the error when logging out:

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-login.php on line 368

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-login.php on line 380

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 697

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 698

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 699

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 700

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 701

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 702

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 705

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 706

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 707

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 708

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 711

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 712

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 713

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 714

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 876

And here’s the error that’s got instead of redirected to wp-login.php:
Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-includes/pluggable.php on line 876

And last, here’s the error when going straight to wp-login.php

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-login.php on line 368

Warning: Cannot modify header information - headers already sent by (output started at /home/jps/public_html/wp-content/themes/artsatjoondalup/functions.php:1) in /home/jps/public_html/wp-login.php on line 380

Followed by the log-in page.

It was working fine a couple of days ago.