pluggable.php -> Can’t activate or deactivate plugins

I was trying to install a very basic plugin this morning, and when I did, I got the following error:

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-includes/pluggable.php on line 329

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-includes/pluggable.php on line 329

I hit the <back> button and then F5 to refresh, and the plugin is activated.

Next, I just tried to deactivate some others that I already have working, and I get:

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-includes/pluggable.php on line 329

And again, I hit the <back> button and then F5, and it is deactivated.

So at this point, I am not sure what’s going on. I did a search for ‘pluggable.php’, but the most recent info is a few months old and not really relevant. Can anyone offer any suggestions? Thanks in advance!

Update: When I go into an existing post and try to edit anything, then hit ‘save’, the changes save, but I still get the error above. I have to hit <back> and refresh.

Update #2: Now when I log out of admin, I get:

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-login.php on line 12

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-includes/pluggable.php on line 381

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-includes/pluggable.php on line 382

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-includes/pluggable.php on line 383

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-includes/pluggable.php on line 384

Warning: Cannot modify header information – headers already sent by (output started at /home/wcipher/public_html/wheelcipher-net/wp-content/plugins/customizable-post-listings.php:308) in /home/wcipher/public_html/wheelcipher-net/wp-includes/pluggable.php on line 331