How to fix a circular redirect?

Question

PHP

There is a site that received free ssl for the domain, which, accordingly, does not apply to the subdomain, but for the subdomain it seems to be not needed, here the site itself has a link to the subdomain "log in to VK through ..." after clicking on which it turns out cyclic redirect (error 301), did everything with 100% accuracy according to this manual, and without ssl on the main domain everything worked fine. Thank you all in advance, can anyone know what could be the reason for the redirect?

Answer 1

To resolve this issue, automatic redirection of the site to https was disabled in the hosting account settings and htaccess was written in which redirection was configured only for the domain, after which the redirect on the subdomain ceased to function, respectively, this issue was resolved. Thanks to all!

Answer 2

Set a breakpoint and see what happens there with the debugger.
If "oh, the debugger is difficult" - add logging to each sneeze and see the log. Although it's easier to figure out how the debugger works.

Answer 3

htaccess to the studio, most likely there are redirects:
1. for all subdomains and the domain from http to https
2 and for a specific subdomain from https to http