Maybe you should give a schematic, how the LCD and 374 are connected and a piece of code, how you access it.

I have never seen, that P0/P2 are used as memory expansion (MOVX) and as IO-ports at the same time.

So I'm in doubt, that it can work in general.

That it seems working for some month on some boards does not prove anything.


Peter


P.S.:
Writing 0FFh to P0 after a MOVX has no effect, because any MOVX or external programm code fetch set already P0 = 0FFh.