This problem occurred for me sometime within the last month on a what was previously a perfectly working WP site. I followed the advice on another post and re-uploaded the wp-includes directory and that solved the problem.
I had not made any changes to the site, but I did see a few user registrations in since it had last worked. I am wondering if someone intentionally altered my site through a vulnerability.
BTW – that was a WP 2.9.2 site.
