What can cause http://i.imgur.com/Y7mKd1H.jpg in the picture? I've only seen it lately and only in this type of weather (with white overcast). I had a similar issue on the ground when I used ENB's Skylighting, but that's turned off. The only things I've changed lately that might affect it is that I'm using Realistic Clouds and ENB's Bloom (plus I've tweaked some other settings in ENB).

EDIT: Well, it's not caused by Bloom and I also get it with http://i.imgur.com/bYHIJHT.jpg, although it's more visible with a white sky. Also, it's not Realistic Clouds but it's definitely connected to ENB as it's gone when turning it off. It doesn't help to turn off the separate effects (i.e. EnableCloudShadows etc.), but it's gone if I turn off ENB. Weird.