That's the 3d editing plane, sometimes (some bug) the transparancy settings get changed.
Go to View > Grid Options > Grids & Background
Here you can change the colour and transparancy of the grid and background.
You can also turn off the editing plane, I think that is at View > 3d View Mode > Editing Plane Display (it might differ for ArchiCAD versions, currently I'm working in a ArchiCAD15 project).
Hope this helps
Erwin Edel, Project Lead, Leloup Architecten
www.leloup.nlArchiCAD 9-26NED FULL
Windows 10 Pro
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