Product: S5500T version V1R5. FC link, HBA type - FC - two switches betwen host and storage.
The customer tried to remove a host from host group from ISM but he was not able to do that. he received the below message:

The custoemer tried to remove the initiators that belong to the involved host [ISM / HOSTS tab, right click on the host, chose Initiators and remove it/them] but also he received an similar message from ISM like above picture and he was not able to delete/detach the initiator from the host.
So, the problem was when he trid to remove the initiator from the host.