Forwarded ssh on bridge device

Hello,

I hope I can bring this clear to you.

I've got a proxmox 3.0 installation on a machine with 2 network devices.

Both devices are used as two bridges (vmbr0, vmbr1) to my virtual machines. One vm is a firewall.
So one device is pointed to my local network the other one to my DSL-Router and so connected to the Internet.

On my DSL-Router I configured a port forwarding for ssh (tcp 22) to my proxmox server, so I can manage my machine from the Internet, while I'm not at home.

This worked well with the older proxmox versions. With the new version based on Debian 7 I can't get any connection. When I do a network sniffing, I see only SYNC packages but no ACKs.

Is there a difference between Debian 6 and Debian 7 in case of bridge networking?

What can I do to get a connection?

Thanks in advance.
Fokko