Unable to create/edit Channel Fields

I’m having issues editing the preferences and creating new channel fields in the system. When hitting Submit, I get a 404 status on the page (website.com/admin.php?/cp/admin_content/field_edit&S=d7871775ecc7fa083d3d10e6319ce9d0)

The site had been up and running for a few months with no issue. Any idea of what might be happening?

Server error log:

[Fri Aug 05 09:07:27.919429 2016] [autoindex:error] [pid 5205:tid 140578553583360] [client xx.xxx.xx.x:xxxx] AH01276: Cannot serve directory /home/user/public_html/: No matching DirectoryIndex (index.html.var,index.htm,index.html,index.xhtml,index.wml,index.perl,index.pl,index.plx,index.ppl,index.cgi, index.jsp,index.js,index.jp,index.php4,index.php3,index.php,index.phtml,index.shtml,default.htm,default.html, home.htm,index.php5,Default.html,Default.htm,home.html,welcome.html) found, and server-generated directory index forbidden by Options directive