Corrupt data on first login everytime

Hi guys since about a week ago maybe less everytime I open the game via steam it says in the bottom left missing or corrupt data. If i ignore this I can play but then random things like invisible monsters on maps happen, lol. I have to close game, kill steam client and reopen for it to download/validate and this is 100% everytime. Once I’ve done this the game is fine. Any idea/thoughts on how to fix this?