keep bringing to log in page

hello, i'm having a problem with the website i set up with the help of a friend, since i don't know much about websites.
my problem is that i keep being asked to log in whatever i do: i'm in the home page of contao, i want to edit something, upload a picture, write a text or whatever else, once i click on nearly any button to go from one page to another i am constantly redirected to the log in page, and when i finally get to edit something, and i want to save or save and exit, 9 times on 10 the changes i made are not saved and i am redirected to the same page as it was before i edited the content.

i tried on 2 different computers, with 4 browsers –safari, chrome, mozilla and opera– and cookies are activated on each one of them. what could be the problem?
this didn't happen on my friend's computer, but it goes on both my computers. we tried together to find it out, but nothing helped, and it's very frustrating. i also tried to google this but it seems nobody else experienced such thing.

any idea?
thank you very much!

Hm, sounds weird. If I understand you correct, you are able to successfully log in (you see the starting page of the Contao backend) but whenever you start some action, you are losing the login session. So somehow the session cookie doesn't get stored or the IP changes?

What kind of internet connection are you using? I know that some people had troubles with mobile connections because these connections keep changing the IP addresses all the time, so that they lose their login sessions.

You could also check if your browser or antivirus programm deletes session cookies.