Strange message upon startup

[ThreeDays]

Hello, everyone,

I've been getting a strange message lately every time I start up BTB. The message reads 'Wrong local header signature. 0x4F44213c'. After I accept the error message I can get into BTB and work normally, but I'm a bit worried something is going to go KABOOM at the worst possible moment because I haven't paid attention to this problem.

Any ideas?

Cheers.