I spoke too soon!
Same problem again today, with Windows Defender.
[Edit]
I have now made an exception for the Sketchup 17 program file in Windows Defender and it seems to be working.
Windows Key with ‘I’ (to open Settings in Windows 10)
Type ‘Virus’ in the box and click on ‘Virus and threat protection’
Click on ‘manage settings’ under ‘Virus and threat protection settings’
scroll down to ‘Exclusions’ and click on ‘Add or remove exclusions’
click on ‘+ Add an exclusion’
choose ‘file’
navigate the tree to 'C:\Program Files\SketchUp\SketchUp 2017'
choose ‘SketchUp.exe’