Do you still have this problem? For we have a customer with the exact error and have found two work arounds that you might try. 1. Open ArcMap, then open the mxd from within ArcMap. 2. Open ArcMap and log out of ArcGIS Online (within ArcMap), close ArcMap and try to open the mxd. Should this not work, do the same thing, but then log in to ArcGIS Online within ArcMap. Hope this helps!
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