The gray corners indicate that something in your assembly is out of date. Perhaps an individual file or sub-assembly has been changed and therefore the top level assembly is out of date. Most likely a part changed, and since the change occurred, the assembly has not been opened and saved.
Go to the Tools Tab and select Drawing View Tracker from the Assistants Section. Make sure to select the show details icon. Scroll down and you will see a list of the parts in the assembly along with icons to indicate which part is out of date and is at fault. You will also see a list of models that you should open and the order in which you should open them to resovle the problem.
You can simply right click on a filename in the bottom pane and open it from there. Once you have opened and saved the offending file(s) you can update the drawing and the gray corners will disappear.
Search SE Help for drawing view tracker.
Bob Henry Former Manager of Engineering Caterpillar Global Mining America LLC
I often get this with mirrored or copied parts in an assembly. Unless the part is activated and marked "Update Automatically" in the properties dialog it will not update when you change the original part. Even if you just open the original part and re-save it will give you grey corners until you open the part copy and save it.