You close Act! and use the File / Close or the X in the upper right hand corner but the Act!.exe still runs in the „task manager“.
So you can’t restart Act! until this prozess is end manuell.
This could happen if you using screen sharing software like TeamViewer or SamsungMultiscreen.
If you have TeamViewer running on this PC the soultion clould be to add Act! to the QuickConnect list. You can go to the list by „Open TeamViewer->Extras->Option->Advanced->Quickconnect in the line before add write ActSage.exe (Act!.exe for Act! 16 an newer)
If it still not work try to disable TeamViewer.
The Solution here is to disable Multiscreen.<