Invalid Security Token

Hi, I have a problem with security token. I tried almost everything that I could find, but nothing helps.

A have a site "http://demo.dreamind.cz/". But my web hosting, has a weird folder structure, so if I want to access website I need to specify path like this "http://demo.dreamind.cz/domains/demo.dreamind.cz/". At this moment, everything is working great, but it look pretty weird and it is unusable for production. So I changed "custom_base_url" in "system.yaml" and website is looking normal, it is accessible with just "http://demo.dreamind.cz/", but administration isn't working. When I try to log in, "Invalid Security Token" error appears. So I even tried specify "session domain" in "system.yaml", but nothing works. When I delete "custom_base_url", log in is working, but with weird path "http://demo.dreamind.cz/domains/demo.dreamind.cz/".

Can someone help me? I even tried to change .htaccess "RewriteBase", but nothing happens.

Thank you very much for everything!