For anyone else in the same boat, I was able to resolve by redoing the chown and chmod commands listed in the install guide on the mattermost directory. I also had to change the TLSCertFile and TLSKeyFile path references in config.json from relative ("./yourtlsfolder/yourcert") to absolute ("/opt/mattermost/yourtlsfolder/yourcert"). I don’t know why relative paths stopped working on my setup.