I have found the issue that caused this problem, but am posting any way in hopes that perhaps this will help someone else avoid the hair pulling this caused me. Plus it’s happened to me more than once and, since I have a less than perfect memory but I do religiously use this forum, I figure if it happens to me again I’ll have given my future self the answer!
So here’s the situation: a few weeks after a smooth site migration, I started getting reports that certain entries won’t save. Editing the entry and clicking the submit button results in the editor being taken to the site’s page not found page. Turns out that the mod_security settings on the new server are a bit too secure! Having the host modify the rule takes care of the issue.
The rule that triggered mod_security was a rule looking for patterns around the use of the word “from”. Our company does a fair amount of transit agency work and, as you can imagine, they use phrases with the word “from” quite frequently. I’ve had to have the mod_security rule adjusted for every transit agency we’ve worked with; all around the word “from”.
So here’s to hoping this message finds me in the future… or saves someone else from an hour or two of WTF-itis.