Check the permissions on your folders and files. Folders are usually 755 and files are 644. If you see 777 or 666 you will get that error.
Thanks for replying. This wasn't the problem. All permissions were ok.
It wasn't the cron job that was breaking. It was my entire PHPList install. Something associated with the upgrade (at least it started spitting errors when Segonax upgraded), broke the .htaccess file.
I went to the installation FAQ at phplist.com. Came up with this ditty:
If you get a '500 Server Error' when installing, your server probably runs php as a cgi, not as an apache module (also known as phpsuexec). You need to edit the file '/lists/.htaccess' delete or comment out (with a #) the line about 'php_flag magic_quotes_gpc'. e.g.
# php_flag magic_quotes_gpc on
I edited my existing .htaccess file and my install (as well as the ability to run the cron) came back. Hope this helps someone out.