That is the reason^^
When I enter http://www.mysite.com in the url I'm logged in but as soon as I click a link it goes back to http://mysite.com and I'm logged out.

umm I see, so try sticking to the url without www, this is now the standard of the script. I believe on most pages if you enter www in your url you will get automatically redirected to the url without www, though sometimes the index page does not do that(which can be a problem if you log in at index page).

I wrote that a bit misunderstandable.
normally the url is on all pages http://mysite.com and the login doesn't work.
When I enter the 'www' part manually the login works for the current page until I click of a link.

umm I am kinda lost in what you are saying. Anyway, can you provide a link to your site? I want to see what errors are there exactly. It is completely different from what I see from my demo site...

Okay:
http://test78433.test-account.com/mysidia/index

Maybe you can find the error.

Umm I did not find any errors, I registered an account and it worked perfectly fine for me(never logged out).

I found my problem...
It's so stupid... I simply needed to clear my cookies and now it works. And the admincp works, too.

Sorry I should have thought of this earlier. Many thanks for your time and your help.

No worries, I am glad the script is working for you now. Lemme know if you find any bugs or errors, I'll see what I can do to fix them.

Hm, I'm having a similar error, and have no idea how to fix it...

I just finished installing my script, and went to my site to click the login. It gives me:

Fatal error: Class 'AppController' not found in /home/brook/public_html/adopt.php on line 3

And when I click on anything, it gives me "page cannot be found". The ACP seems to be working, I just can't access it :/

So...what do? I'm using webhost x10hosting with subdomain elementfx, if that's needed. Otherwise, the site appears to not be working. I'm using 1.3.3, too.

Gah, I have too many questions -.-;

If you are getting the AppController not found error, chances are url rewrite aint working properly. You may try reinstall the script, but I doubt it will help. Anyway x10hosting seems to have some trouble with url rewrite for a few people, there is a chance that you have to fix up the .htaccess file a bit to get it to work.