Ok, there were a couple of things.
This Nagios installation is on a virtual machine, and I mistakenly set it to fixed disk size. It filled up on me, and corrupted my databases. I started over from scratch with a new VM and new Nagios installation. I forgot to edit my /etc/hosts file, so Nagios had nothing to point to when referring to "localhost." Fixed that problem. I had to fill out the forms for the service on 192.168.20.25, and assign it a host again. Once I did those things, and applied the configuration again, it worked!
Thank you for showing me that "Delete/Write/Verify" method. That showed me a detailed enough error where I could debug properly. Along with the previously mentioned error, it also showed me
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Could not find any host matching localhost