This occurs when I attempt to connect sdb1 or sdb2 to an AppVM. The disk contains two partitions, both LUKS encrypted. This error only sometimes occurs. It seems like it doesn’t occur after I physically reconnect the disk and wait a few seconds.
This is the error in dom0 journal. I typed this out manually.
dom0 qubesd[2778]: WARNING: protocol error for call b'admin.vm.device.block.Attach'+b'dom0+sdb1+0bda+9210+830700000022+u080650u80662+1.0+1' (b'dom0' > b'vault') with payload of 147 bytes