I think I've found a solution.
Try changing the path in ImageSource to
pack://application:,,,/{AssemblyName};component/{anyFolder/image.png}.
In your case, it will look like this:
pack://application:,,,/{AssemblyName};component/Image/NextDay.png
where {AssemblyName} is the name of your project/solution.
This solved the problem for me.

By the way, for your information, if you use Image instead of ImageBrush, there will be no debugging errors, although there will be no image either. But at RunTime everything will work as it does with ImageBrush.

This problem froze my project for two days. I spent two whole days looking for a solution and found this. I doubt this is the right solution, so if anyone finds something better, please let me know.