I had (I think) version 1.23 because that's what the auto-installer on my host put there. So I uploaded the files for 1.31 and ran the update.php script. The site seemed to run fine until I tried to save an edit, at which point it gave me this error:
I disabled all the extensions, which stopped the error, and then re-enabled them one by one, and discovered that the two that caused errors were Spam Blacklist and Title Blacklist. I confirmed that they were the most recent versions, so that isn't the problem. Is anyone aware of this issue?