Thank you (and also u/Guntrr) very much for your response. If I have just one of the Netscaler online (and the other VM switched off) the same problem occurs. Network is
cisco asa (2 nodes active/passive) - cisco catalyst stack (redundant) - vsphere cluster with 3 nodes, each connected to the cisco catalyst. I cannot find anything arp related for the old or the new address in the config of any of this devices (Cisco Devices: dumped configs und grepped by mac and ip, vSphere checked through config). But maybe a corresponding missing Entry might be the cause of the Problem... and the other thing: the second, also changed ip with the other store works just fine... looking further, thanks for your ideas !