Hi Tea-Rexx -
If you are using Dynamic GI then you have enabled LPVs, correct? I ran a test and just believe that you have turned your skylight down too far, try using 0.009 instead, its too close to 0 for float point error and the engine will try to read it as zero.
I have attached my test project for your to look through, if you see an issue that differs from your setup please correct in the project and rezip it back up here. To switch from night to day Press K.
Thank You
Eric Ketchum