It was me - stupid self - but also a nasty Bug in SRM 5.5
I have some VMs configured with access rights only for me (my AD-domain account) and for all others including all at vsphere.local NO ACCESS.
So it's correct that SRM cannot read out their inventorymappings and so not being able to protect them.
I gave the SRM acount access rights and the error was gone.
But: That the SRM-service itself dies on that isn't an behavior I would expect in this situation.
Seems to me being a bug.
... solved with workaround ...