RHEV-M not able to access iSCSI LUN previously added during installation

Hi there

Today i started using RHEV and it's quite nice so far.
I still have storage related issues which i cannot resolve by myself.

I'm using this document as installation reference:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Virtualization/3.6/html/Self-Hosted_Engine_Guide/Configuring_Self-Hosted_Engine_on_Red_Hat_Enterprise_Virtualization_Hypervisor.html

My main problem now is that i cannot access the iSCSI Lun i added during the self-hosted_engine installation of the RHEV-M.

When i access the web-interface and try to add storage by myself (i already wondered why it's not added automatically, since i added it during the RHEV-M installation) i only get a window, that it's already in use where i can check a box to confirm but after that i get another window with the message that the volume-group couldn't get created.

Thanks in advice,
Tim