Kurt Buhler broke it and now needs to buy it:
The “grey box of death” in Power BI is unfortunately a common sight for anyone creating reports. When you see this grey error box, it means that the visual can’t obtain a valid result. If that error says “Something is wrong with one or more fields”, then there’s several possible reasons for this:
Click through for those reasons, as well as how to fix the problem. Kurt shows this both in video form and written out, so check it out.