Sandeep gave me a call in the evening today and told me that he is getting this error when he tries to launch forms from self service interface:
Oracle.Forms.Engine.Main Not Found
Metalink Note 465063.1 has this error and advises this:
-- To implement the solution, please execute the following steps::
1. Go to control panel. Jinitiator 184.108.40.206
2. Click on the proxy TAB
3. Enter Proxy name and port
hardcode proxy name in the browser.
It was indeed because of the proxy. Sandeep confirmed this after checking with another user.