I have 2 SSDs.
One main SSD has Qubes and all AppVMs. The second SSD (thin pool) is for storage for one AppVM with a large amount of data in it. After the upgrade, it’s not possible to start the AppVM from the second SSD. As described above, the vm-XXXX-private file is present on the second SSD, but the AppVM, after the upgrade, searches for it at some other location (I guess).