I found this UCR-Variable while researching about the error but then totaly forgot about it.
This did the job, thank’s again!
Just for the records:
I set intel_iommu=off through the Management console (UCR-Registry).
This parameter is entered in grub under GRUB_CMDLINE_LINUX="intel_iommu=off"instead of appending it to GRUB_CMDLINE_LINUX_DEFAULT, so in /proc/cmdline it will appear before the DEFAULT-entries.