The problem seems to be with the APN's. My APN list is now empty. Before, my custom APN had been there until this problem came about. Any APN I try to create and save doesn't even show up in the list. Also, when I try to create a new APN, the MCC and MNC fields default to "Not set" instead of 310 and 410 respectively.
I am on an unmodified, stock Nexus 5. If you know of a solution, don't hesitate to post.
Thanks in advance.