oddend

Here is some more information.

When you create a new post and enter a title, wp-admin/admin-ajax.php is called three times.

The first is the autosave action which succeeds
The second is the sample-permalink which also succeeds
The third is the autosave-generate-nonces which fails with a 500 internal server error

Then when you publish wp-admin/post.php fails with a 403 forbidden error, probably because the nonce is missing.

When the plugin is disabled, the autosave-generate-nonces call succeeds as expected.

Correct, when I disable the plugin the problem goes away. The weird thing is that it occurs only for new posts. I have no problem saving or publishing existing posts.