FYI: During the day, I made edits to the Settings->Language files. I wouldn't think that would have anything to do with the problem, but I thought I would mention it.
Earlier, I attempted to disable Captcha, but since the edits didn't work, I reverted back to the old join.php and join_index.html.
Then, this issue cropped up.
Afterward, I downloaded a fresh copy of the oxwall installation files, and uploaded join.php and join_index.html to make sure they were the right files. That didn't help the problem.
NEW INFORMATION:
I now see that there people joining successfully. I've tried joining with a test account and it won't let me. Is there some kind of feature that detects your IP and won't let you sign up multiple accounts within a given timeframe? Can't figure out why others are joining and I can't.