Ahah 403 error

Issue Summary

The error occurs with both the dev version and the production version.

Drupal 6.19 (I know, I know) and using phpqrcode-2010100721, google disabled. Except for the core, all the other modules are up to date. The problem is with the Mobile Code Generator block. I installed it on a particular page. I also installed Admin Roles, so I have users as follows; anonymous, authenticated, authenticated + administrator, admin.

When I access this block, the admin user works fine. I can input a URL and it generates the correct QR code.

All the other users fail, including the user with administrator role (all permissions). I get this error: An HTTP error 403 occurred. /ahah/mobile_codes/generator

I've set all file and directory permissions in the web root owned by the web user to rule out an Apache problem.

Any help would be appreciated.