Cannot create new post in 3.3

I am unable to create new posts in my blog. The timeline:

1. I upgraded to 3.3
2. I installed the WordPress Importer plugin and tested importing some ancient posts from another blog. It worked fine the first time, but when I went back to import more, I got the “Sorry, there has been an error. The file does not exist, please try again.” error. I have reported that on forum for that plugin. [This may or may not have anything to do with the problem.]
3. The next day, I tried to make a new post, and got the

“Are you sure you want to do this?

Please try again.”
Error.

I am using it in multi-site mode, and am able to post on the other blogs.

I have tried:

1. Deleting the WordPress Importer plugin

2. Deactivating ALL plugins via phpMyAdm and then via ftp

3. Switched the theme to 2011

4. Reinstalled the 3.3 upgrade, using first the automatic and then the manual process.

I get the same behavior after each of these fix attempts.

Since I did not try to post between the 3.3 upgrade and using the Importer plugin, I do not know which of these caused the issue.

Thanks!