I've been Googling this for a while:
I was on 2.8 when this bug occurred, so first thing I did was finally upgrade to 3.1. It didn't fix anything.
Next thing I did was disable all the "Update Services". Didn't fix.
Then I tried disabling all plugins. Didn't fix.
Occasionally it WILL post/publish/update, but only part of the post, leading me to believe all the data is not being submitted (?).
Another interesting thing to note that has happened - around the same time this started happening, my admin password stopped working and I was unable to login. Email recovery wasn't working either. So I had to manually change my password using MySQL.