Sorry, but still a problem: the form and submit button in customers.php is correct, the master password is indicated, but when I click on the Place Order link, I get redirected directly to the index.php?main_page=login page, with no information in the email and password fields...

Indeed, if I directly type the following URL :
index.php?main_page=login_as_customer

I always finish on that page:
index.php?main_page=login

The tpl_login_as_customer_default.php file has been placed in both my template and the default_template...

Thanks,

sanji