Did you try to select a folder? Click on the little down arrow at the right side of the "Select Folder" box.
I have had no problems saving to source box.
@donmthomas, Áine is correct: the "save" button is greyed out for you because you don't have a destination folder chosen, so it doesn't know where to put it. Choose a folder and you'll be able to save.
Both above statements are not correct. I have done nothing about choosing a folder and again have the blue button working when I want to save to my source box. Thank you to someone for making the source box work again.
Now the source box is not working again. In using the source box for years I have never had to make a folder. Guess I will just STOP using it, if it is not going to work like it has always done in the past. I have always used the source box to temporarily hold a source, and once the source is put into the Family Tree, I delete the source out of my source box, so no need to have any folders in that my source box is empty unless I am using it.
You don't need to make a folder. You just need to choose one. "Home" works just fine.