Is this true also for zc 1.5.1?
dr. Byte said:
Zen Cart v1.3.x currently checks for the following conditions based on $_SERVER variables available to PHP. If any of these tests passes, Zen Cart treats the page as SSL:
HTTPS = 'on' or '1'
HTTP_X_FORWARDED_BY = 'SSL'
HTTP_X_FORWARDED_HOST = 'SSL' (and v1.4 and newer will check whether this matches the HTTPS_SERVER name too)
SCRIPT_URI is passed and contains 'https:'
SERVER_PORT = 443

(this all depends on ENABLE_SSL being set to 'true' in your configure.php file, which you've already confirmed to be the case)
thanks.
enzo