I succesfully used the upgrade tool to upgrade Qubes from R4.0 to R4.1
Now, when I want to start my Windows 7 VM’s, I get the error that libxenlight cannot create the new domain. I’ve found this issue: Migrate backups of Windows VMs created under Qubes R4.0 to R4.1 · Issue #7088 · QubesOS/qubes-issues · GitHub
However, that is about imported backups. Also, in that issue, the Windows 7 VM’s actually start for a short time before crashing, instead of the libxenlight error I have.
My Windows 7 VM’s were actually restored backups from Qubes R3.2 into R4.0, as at that time, there was no QWT for R4.0 yet. Uninstalling QWT in Windows crashed the VM and made it unusable, so I never upgraded to a newer QWT, and the installed QWT version is still the latest from R3.2