i found ipmi call traces in log
Code:... [ 243.111784] Workqueue: events redo_bmc_reg [ipmi_msghandler] ...
My system is very old Dell C6100. It was quite stable before update. In test VM update from 5.4 to 6 with openvswitch goes flawlessly
Had a very similar problem also with a Dell C6100 that later on in the stack trace showed "Code: Bad RIP value.". This was the only thread I found, so for anyone looking this might be helpful in the future: In the BIOS, set the BMC to use a dedicated MAC instead of sharing it.
Afterwards, booting the system didn't block for a longer time. This also prevented starting the Proxmox installer before.