By Miles Nicholson
One possible reason to cause the error message is a side by side installation of AutoCAD and AutoCAD Electrical (both should be 2016).
On opening AutoCAD Electrical 2016, the error message “Synchronization was not successfully completed” appears.
Select Close and the error message will disappear until you open another electrical project from your Projects Manager within AutoCAD Electrical.
The message relates to the synchronisation between AutoCAD Electrical and Inventor.
To stop the message, simply type at the Command line “AEEMXDATAUPDATEMODE” and set to “1”. This basically changes the electromechanical synchronisation to run in the foreground rather than the background.