Error: Mgmt. IP address(es) and virtual IP address need same mask and subnet

Hello,

my problem is that I want to change the IP of three S4810 Force10 Switches where two of them buliding a stackunit and one is an edge switch. For the edge switch I could easily change the IP of the ManagmentEthernet, but for the stackunit I always get the Error Message:

% Error: Mgmt. IP address(es) and virtual IP address need same mask and subnet

This one I get also when I try to change the virtual IP. It also doesn't matter whether the stackunit is connected or not.

What can I do?

Example: I want to change one of the Mgmt. Ethernets of the stackunit from 130.170.5.241/24  to 130.170.119.251/24.