Plesk 11.x / Linux :: No Access Because Server Redirects To Its Hostname

I enter one of my IPs or the hostname belonging to this IP, followed by the Plesk port, and I get redirected to the "main" hostname of the server, which was set in Plesk. But at this moment that hostname is used by another server (my old server) and so I'm redirected to the other server and cannot access Plesk on the new server.

I don't know why Plesk reacts like this, because it did not on the old server. Here the problem more detailed:

I'm just configuring my new server with Plesk 11. My old server uses Plesk 10. On my old server / in Plesk 10 I have the following scenario:

- I have several IPs (10)
- Each IP is used for a different domain
- Each IP has it's domain as the reverse entry (configured outside of the server/Plesk)
- Each IP is set to "Dedicated" in Plesk
- I set Plesk to listen only on one of these IPs (and different port)
- Several domains are created in Plesk, but not a domain for my Plesk IP
- The server's name is one of the domains (required for mails not being recognized as spam)
- The hostname of the Plesk IP is one assigned by my provider (something like x-x-x-x.xyzservers.com)
- Entering the IP of Plesk redirects to the "Plesk hostname"
=> Plesk can be accessed by using x-x-x-x.xyzservers.com:myport

And under Plesk 11 (here comes the problem):

- Still several IPs (but less: 4)
- One IP should be used for Plesk, one for domain X, one for domain Y and the last one for the other domains
- Reverse entries are x-x-x-x.xyzservers.com for Plesk IP, domain X/Y for two other IPs and the main domain for the last IP
- Plesk IP and the two single domain IPs are configured as dedicated, the last one as shared
- At this point I did not configure any domains in Plesk
- Server's name is the same as my old server (the main domain)
=> Entering any IP with the Plesk port redirects to the servers hostname, e.g. my main domain, which is still on the old server.
=> Result: I cannot access Plesk!

Of course I could just change the hostname of the server via SSH (just have to find out how to do this), but why does Plesk react in another way now? Or is the problem that I cannot change Plesk to listen on only one IP?