I just went online and found that most, but not all, of the skins redded-out and showing the default skin. Some skins were working OK but these were all older ones. I then realised I had not updated the online skins for a long time so I re-downloaded all of them from the
AH Skins Site. Skins now all seem to be working correctly online, with none showing redded-out on the various aircraft I checked. Perhaps those still having problems might try downloading some skins again to see if that fixes things.