ACP Login Problems

Hey guys, I've set my new IPB Forum up but got a small problem:

Everything worked fine, I installed a new theme and checked it out - works perfect. I made the default Design from IPB invisible and 2-3 minutes later I couldn't login into the ACP anymore.

I click on Admin - type in my Username & Password - Page open but nothing happens.

But when I copy the link of the admin panel, for example when I am editing the Member groups and paste it, it works. So I was in the ACP again and everything works, except if I click on Dashboard or on the picture "IPS Community Admin". Then it's an empty page again.

I don't want to login always with the weird link, I want to login when I click on Admin, but as I said the problem is an empty page. I think I made a mistake with the index but I don't know how to fix it.

Anyone know how to fix it? Sorry when I explained shit, my english sucks..

Edit:

This is the Link which appear when the empty page comes:

http://mydomain.com/admin/index.php?adse...8f2bd8&

But if I login for example with this, it works

http://mydomain.com/admin/index.php?adse...pp=members

(I removed my domain because it's not public yet)