Thanks so much for your quick response!
All of these suggestions are really helpful. I think our problem is too many requests, as I’ve been pulling up multiple pages at once in order to review them. I’ve asked my client if they’ve also been doing this, because if so, it sounds like this is the source of the issue.
Thankfully it wouldn’t be a problem for a regular website visitor, so if this is the issue, all is well. Otherwise, I’ll try a page caching plugin and find that error code snippet and will get back to you.