BUG: Safecracker (Channel Form) form neither stores data nor generates error?

I’m trying to upgrade a form from EE1 to EE2. I worked in EE1. In EE2, it returns nothing, just the form itself, empty. No error, no change to the database.

I’ve also tried using the Ajax Channel Form code provided in the docs. Same thing – the form returns, empty, and there’s no error message, nothing added to the database.

Removed all third party extensions, this is a stock EE2.7.1 install.

Known bug?

How to begin debugging this?