I'am back in the game 
My full installer VM BigSur 11.3b4 (20E5195f) on BigSur 11.2.3 host was successfully updated to 11.3b5 (20E5217a) today.
My upgraded VM BigSur 11.3b1 (20E5172i) on BugSur 11.2.3 host was successfully updated to 11.3.b5 (20E5217a) today.
But I don't know why this happened :-).