Is the sun behind your target? And not very high above the horizon?
I had very interesting effect once, sun was as i described, behind the target and not very high, and what ive seen through the scope is complete white, like i was looking at the snow field. And i had sunshade. It turns out, that sun was reflecting from polished barrel surface right at the scope.
Also ive noticed, if the sun lights on large object, like a bush, located near or in front of target, but outside field of view, i see like a reflection othat, all picture slightly green.
from Russia with love.