Sudden 500 server error, though no changes were made

Hi! Our site (http://brill.gitlab.io/) is suddenly giving a 500 server error. There have been no new commits over the past few months, and the site was working fine earlier today… Is there any explanation for the site not working? I note there was a note on a redis failover about 16 hours ago, but our site appeared to be working more recently than that.

Many thanks!