I found quite weird bug, whenever tutorial question mark apear, whole game window goes black.
If you open any other interface window(inventory/clan window/etc), you can temporarily see the screen again, but as long as tutorial question mark is present, you'll go back to black once you close the interface.
I tested it on clean instalation instance of L2 interlude, with only cache size l2.ini modification and with and without background performance mod(fire.dll, nosleep.dll).
Tested on W10.