To fix this problem for the time being go into the diablo IV directory and rename dstorage.dll to dstorage.dllbackup and the problem will go away.
I don’t recommend deleting the .dll file as once an offical patch is put out you may need / want too put it back.
All I know is by doing this it 100% fix the logging in / initial loading issues I had and allowed me to move the game back to my must faster nvme drive.
Must have something to do with directstorage and very fast drives. Maybe the game is just loading too fast on startup / logging in and its breaking something with Diablo IVs multithreading.