I have confirmed only one instance of Graylog is running. And using lsof I have confirmed that no other process is using the .lock file.
The journal directory is not on its own disk partition.
But that reminded me of something I read in my prior research: someone mentioned that having any extra files in the journal folder would cause this error. I didn’t have any extra files, but I did rename the journal folder as a test. Graylog starts up correctly now.
So now the question becomes: how can I process the messages in the journal file I moved? Can I just move the log and index files in to the correct messagejournal-0 directory?