User registration silently fails - how to debug? On our Drupal 7 site, user registration silently fails.
Drupal doesn't even reach the form's validation functions.

What we checked:


disabling javascript didn't help. 
Removing installed modules related to registration such as : logintoboggan, user_registrationpassword didn't help. 
Error logs show nothing.
The user module is of course enabled, people are able to login as usual.


Please advise where to put tests to see where execution is stopped. 

Thanks in advance

Amnon
Recommended steps to trace the error:


Look at watchdog (which you did)
Look at the systems php and apache (or whatever you use) error logs
Take a local copy of the site and start digging


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.