I am assuming this tank has a screen top? Also what kind of heating do you use during the daytime?

In my experience, a glass tank with a screen top and heat lamps usually has LOW humidity... have you double checked that whatever hygrometer you're using isn't malfunctioning (i.e. put an extra one in there to double check that it's correct)? If it is indeed the hygrometer that is wrong, it could save you a lot of trouble by just replacing it.

Also, have you compared the humidity inside the tank with that in your house? If your house humidity spiked without you noticing (due to weather changes), this could have an impact.