How can I change the path for the admin folder? I want to rename this folder but found nothing in config.php and constants.php to define the new path. When I just rename the admin folder I get an error.

Yes, it may not be that easy.

But, why do you want to change the path of the admin ?

It's a aspect of security. I don't want that everybody knows that he can find the admin login under /admin.
I came from MyBB and there I renamed my /admin to something like /iop8jm34f5fIUZSHhiulsao89p549ß03,
and /admin was just an dummy honeypot with no function.

We will do something about this in the next version so that the name can be easily changed without creating any problems.