Avatars plugin stopped working

The avatars plugin appears to have stopped working on my site. Default avatars show up fine, and the upload process allows me to upload and crop the image but beyond that, I just get a broken image icon where the cropped avatar should be.

The uploaded files appear in wp-content/avatars but do not appear on the live site. My .htaccess file is correct.

This applies to both blog and user avatars. Plugin version is 3.3.3 on WPMU 2.7.1.

Thank you.