Hi @dyle,
looks like the problem was already reported to the kernel developers in 2018. However, no fix is available for it.
https://bugzilla.kernel.org/show_bug.cgi?id=199733
The workaround would be to disable IOMMU in the BIOS of your machine. This should not have any functional consequences, except if you have other VMs running on top of IPFire (discouraged for security reasons anyway), particular if PCI device pass-through is used.
Sorry to disappoint, and best regards,
Peter Müller