Entire Mac scan

Jean, 

Sorry to hear that the issue persists, we appreciate your efforts in trying to fix the issue. We request you to uninstall and reinstall your AVG Antivirus for Mac and check with the issue. Make sure that you removed all the old traces of AVG completely. 

To uninstall AVG 15 -
Open Finder > Applications > right click on AVG AntiVirus.app > select Show package contents > open Contents folder > double click on Uninstaller.app.

To Uninstall AVG 17
Run uninstallation from /Library/Application Support/AVGAntivirus/components/uninstall/com.avg.uninstall.app

I've done as you suggest but the Antivirus still crashed.  It crashed at 65% when it was on
   Users/…/ Library/Mobile D…
Have you any other suggestions please.

Hello Jean,
I'm sorry to hear about this. In this case, I request you to reboot your Mac computer once and run a whole computer scan with AVG. Keep us informed about the status.
Best regards,
Alok.

My Antivirus for Mac crashes before it completes it's full scan.  It is up to date and there are no items in quarantine…  Any ideas what's wrong please.

Hello Jean,
I'm sorry to hear about this. In this case, I request you to reboot your Mac computer once and run a whole computer scan with AVG. Keep us informed about the status.
Best regards,
Alok.