Bit of a weird issue but it seems you can manage to get stuck in paste mode with no copy area. Not allowing you to place any new blocks as you're pasting empty areas instead.
By just using copy brush and confirming a selection, your next click seems to clear the copy area as both the **Paste** and **Save** button disable themselves (which only happens if the copy area doesn't exist, or is 0 blocks big).
Another issue is that **Save** acts like it is **Paste**. Using that button won't save the copy area, but instead enter paste mode.