This is weird.
Everything was working beautifully.
I was checking to see if I had a plugin conflict with something else, and so I just turned off all plugin's (including relevanssi) (disabled them). Then I turned them all back on after doing the check.
Later in the day, I noticed that the relevanssi search was not highlighting the words like I had it set. I thought that maybe disabling it and then re-enabling it reset the settings. So I checked. Everything normal.
I clicked the "Build the Index" button and I got hit with this:
500 Server Error
A misconfiguration on the server caused a hiccup. Check the server logs, fix the problem, then try again.
I have checked the server logs and there is nothing in them.
I have uninstalled and reinstalled the plugin, and I still get the same thing.