I have read most of this topic's entries since my last post and I can say some of your findings are not correct.
I have seen these blocks in vanilla, in slightly modded (xml by hand) games, in nitrogen generated worlds and in darkness falls. I have an Nvidia card and ssd drive.
I have tried pois which switches this off and using it again turns the problem on again. These blocks are completely immaterial, you can walk and shoot through them and when you're inside, you can see the outside world (they are one-sided).
These blocks are chunk based - I experience them in building X, ride my motorcycle to a distant trader (no blocks during the travel), return back and the blocks are gone.
I have read about a memory leak the game has, that is correct. Most often these days, when I quit the game, I restart the pc as well. Sometimes I do that before I start the session as well, since the leak occurs when exiting game, not the actual program itself (continue game, exit, continue game, exit, likely crash from insufficient memory). I guess this has nothing to do with these blocks but it is just an educated guess. No proof here.
I'm starting to believe this might actually have something to do with hardware graphic card, although this is not backed up by proofs either. I used to have a weird glitch (which by the way occurred to me exactly once while playing a19) where one vertex of a triangle moves away to infinity, stretching that rectangle all the way until it's no longer visible in the distance. Again, completely phantom. In the past, I solved this issue by returning a faulty graphic card and properly cooling it. Might it be the card is too hot and fails to remove some objects?
Anyway, this needs to go, since when you are a server and four people connect to you, it's not really a solution to restart the game. Given the eternity it takes to load the game... this has to be fixed. Sooner than we get another fat cop model. Please.