Somehow, at some point in the last couple of days, my Nextcloud docker started having issues. Monday morning things were fine, but at some point in the afternoon we started getting a 504 error.
Nothing had been changed in Nextcloud or Unraid, so I'm at a loss as to what happened. Since the problem came up, I made sure that all of my dockers are up to date, as well as Unraid, I've restarted the server, but we're still seeing the problem.