We recently faced problems with application processes on demand.
The problem is that the ajax requests always fail resulting in the error mentioned in the title when:
1. You open more than 1 instance of the same application and call a on demand process from a older instance. Basically you open the same application twice in the same broswer and computer, then you use a on demand process on the first opened, this will result in the call fail with the Not Found error.
2. You call a on demand process from another application. For example you use in a application id say 1000 the on demand processes stored in the application id 2000, this will result also in the Not Found error.
My basic need now is to skip authentication in some application processes, because they are called very frequently and this has too much impact on performances.
I used to have this solution in the past (apex 4.0), using a second application with no authentication I could call on demand processes avoiding the authentication process.
I am quite sure that this hasn't been working since we upgraded to apex 4.2