[Plugin: Simple Image Sizes] Error 500 on regenerate.

I am trying to regenerate all images from the media settings file. I get an error 500 when it runs from a pinkish drop-down at the top. it says
Error requesting page http://mysite.com/wp-admin/admin-ajax.php, status 500 : Internal Server Error.

Are there any settings that I should have enabled with my SQL or Apache settings to run this. Everything else about this plugin is working fine.

THanks!

http://wordpress.org/extend/plugins/simple-image-sizes/