I'm currently trying to move a Joomla site to another host. For testing I created a duplicate of the site with a different domain (I don't know if this has anything to do with my problem).
The problem is that I can't login anymore. The administrator login (/administrator) works fine, but I cannot login with the login form on the Joomla site itself. I enter my credentials correctly, but then Joomla redirects me back to the login page.
I looked into the database into the session table, and there is an entry with my username. But also now there is another guest-entry, with the same
sessionId as the cookie in my browser now has.
If I check "remember login" in the login form, then I can login. But every time I load a page of my Joomla site, a new session is created in the database (because Joomla "forgets" my session every time and the "remember me" creates a new session for me every time, I think). And I then cannot logout anymore.
Does anyone have an idea where the problem is, and what I can do? By the way I'm very new to Joomla (I have to maintain an existing site). We're using Joomla 3.7.3 and PHP 5.6.40 (I cannot update because of too much customisation from my predecessor, currently I'm creating a completely new website).