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Virtual Machines -> select VM -> Virtual Disks -> New -> for some VMs, the New Virtual Disk dialog shows a "Close" button instead of OK/Cancel buttons. This means that it is impossible to add new disks to these VMs.
The problem was that the domain was Inactive. There should be a red error that informs the user of why adding a new disk is impossible.
Once disks on multiple storage domains will be supported, there won't be a need in such error message; an error message will be needed in case there are no active storage domains in the VM's data-center; this message should already exist now (if it doesn't exist, it is a bug as well).
git commit 8208e4a6a8336fcb6eb453a02d4ce35d05543232
Checked on upstream, last commit c9fe3861562df5dbbeebb14345ad3b9e27c5091e.