Invalid form key Magento 1.9.3.1 clone installation https

[dedu_office]

LS.,

I am running CE 1.9.3.1.
I have a form key problem with loging into my adminpanel and in the frontoffice i have an issue with loging in and with making a new account.
I made a clone of a shop i am almost finished with and woth this clone i wanted to build a new site.
This clone has on another domainname, but is running on the server.

Is there a way to get rid of this invalid form key issue?

I didnt have this problem when i still had CE 1.9.2. and was entering in the clone.
My site is running https, is this a cause of the problem too?

I tried a lot, flushing cahches, refreshing, clearing history of the browser, try different kinds of browsers.
Nothing did the trick.
And i am not using Varnish or any other caching service.

Please give me some hints were to look.
What files, which alterations.
What and where to change settings in Myphpadmin.

I am looking forward to any replies regarding this issue.

[Francis Kim]

Sometimes the invalid form key arise from incorrectly set times on the server - have you checked this? What's your cookie lifetime set to?

[dedu_office]

Cookies are not the problem, i set it before i cloned it to 360 seconds and tried it seferal times, but no result in the frontend nor in the backend.

Is there any code i can put of? (Code of the invalid form key)

It is a bit wierd..... in 1.9.2.4 it worked without problems but in 1.9.3.1 i get this invalid key code.

The time zone cant be an issue, it is GMT +1 everywhere also on my server.