Each time Arsenal takes a shot, it temporarily disables the auto-focus (AF) functionality of your camera's shutter button. Arsenal does this to prevent refocusing when you trigger the shutter, since the focus is set in the Arsenal app.
Arsenal makes this adjustment by changing one of your camera's custom controls. Under normal operation, Arsenal then re-enables AF after each shot. This ensures your camera behaves as expected without Arsenal.
If Arsenal loses its connection to your camera before re-enabling the setting (for example, in the middle of a stacking operation), your camera's AF won't work as expected.
Rest assured, there is a simple fix for this issue:
Open your camera's settings menu
Navigate to the screen that allows you to assign custom functions to camera buttons. On a Canon, this is usually called "Custom Controls".
Select the custom control for your shutter release button and change the setting back to "AF + AE" (or just "AF" if you prefer).
Save the setting, then try shooting again. Also double check that your lens is set to AF.
The following video illustrates this fix being performed on a Canon 7D mark ii:
If this doesn't work, try reconnecting Arsenal to your camera, taking a shot, and then powering down and disconnecting Arsenal.
Thanks, and apologies for the inconvenience!