On our Drupal 7 site, user registration silently fails. Drupal doesn't even reach the form's validation functions.
What we checked:
Please advise where to put tests to see where execution is stopped.
Thanks in advance
Recommended steps to trace the error:
You can find your system logs (php and apache) by looking at the php info under "Admin > Reports > Status > PHP".
For local debugging: http://ratatosk.net/drupal/tutorials/debugging-drupal.html
My first point of entry would be the form definition. You could try for example to add an
#after_build callback that get's called once the form has been fully build, and then check which validation and submit handlers are registered.
In your case I would suppose though, that you don't need to go that far. It's probably pretty basic error that will show of in the system logs.