Who faced, the authorization of the admin panel (invalid form key) and the user (just a redirect) Magento 1.9.* does not work?

Question

Magento

The problem - when authorizing in the admin panel - writes invalid form key.
When a user logs in, after entering authorization. data - redirect to the same page.
It’s normal to debug where something is lost, I can’t. locally everything works fine.
(Ps for LAN to get as close as possible, download the nearest backup, (files + base) )
The problem appears only on a live one.
Installed, enabled FPC and regular caches.
Perhaps some kind of cache on the hosting, but there is no way to check it.
A few more times the situation slipped when, after another attempt, something changed somewhere (mostly played with cookie configs), because. I'm guessing that's the problem.
I clear the browser cache, magenta, reset the FPC and for example it only works in mozilla, but not in google.
I googled a lot, tried it .. but there are no options at all ((
Maybe someone came across ... I would be glad for any ideas.

Answer 1

check whether cookies and session are generated and whether there is a formkey parameter in the authorization form