I was one of those who had this exact problem (I posted above), but I have now solved this for myself. I used a brute force method that may not be suitable for anyone else.
I'll describe the situation and my version and you can decide if it applies to you. There are a lot of "can't save" issues here on the forums so I wouldn't assume that my solution will work for you, plus for many of you it may not be practical.
Symptoms: After over a year of blogging I had the same problem described in the original post: When I enter *anything* in a new post, and try to save, publish, or save and continue editing, I was redirected to the blog main page.
This happened with any user who had the right to save.
I also found that if I tried to edit an existing user in any way that I had the same redirect immediately upon clicking "Update User." Adding a user did NOT result in the problem.
I'm on version 2.2.1, but I believe that if you are having the exact same problem that I had, the version doesn't matter. (I could be wrong about that.)
I hadn't changed ANYTHING, but I realize that something must have changed somehow, so I started looking at the files on the server, and saw that the date of my index.php file had been updated. It was now marked as having changed on the same day that I started having the problem. All the other files still had a date from back in 2007, when I did the initial install.
In index.php I found that the license text had been changed by someone.
There's a line that I think is supposed to say:
but instead it said:
That looks like somebody hacking my file and changing the license to some site related to Search Engine Optimization (SEO) tools.
So I changed it back, but that didn't help. It might not even be relevant to the problem.
Bur since I now knew that at least ONE file had been changed without my knowledge, I realized that ANY file could have been changed.
So I called my hosting provider and asked them to restore my entire WordPress blog folder and all subfolders from the backup they had of the day before the problem happened.
And that solved the problem.
So I don't actually know what the problem was, and I don't know if any of you have the same problem.
But IF this is a hacking issue, it would explain why it suddenly stopped working for several people on several different versions of WordPress.
The solution I used worked for me because I hadn't recently made any changes to theme or settings or options in WordPress, and so restoring from a backup that was a couple of days old didn't change anything on my blog. And in my setup, the database that actually holds the blog entries is on a different server altogether and wasn't affected by the restore of the WordPress folder and sub-folders, so I didn't lose any blogs.
Just in case, I took a database backup and copied all the WordPress files that were going to be restored before my provider did the restore.
I would still love to hear any theories about WHICH file or files might have been changed to cause the problem.
My hosting provider is Pair Networks. I'd also be interested to hear if other Pair subscribers were affected.