I also have problems upgrading from 8.1.5-6 to latest 9.0.0-6 ... I have three Windows (Win10, Win XP,..) vm's which run into a BSOD bootloop when starting with qemu 9. Everything is fine after reverting to 8.1.5-6.
root@minimaster ~ # qm config 101
agent: 0
balloon: 0
boot: order=ide0;net0...