Windows Defender is fine and does not (should not) cause problems.
It’s not good that you can’t open the resources folder from the Settings → Manage Resources subwindow and that really needs advice from a developer.
Assuming it was installed from the Windows Store, someone else has been having resource folder access problems with that one, as described here:
For an ‘ordinary’ installation, the resources folder is at:
C:\Users\ {user-name} \AppData\Roaming\krita
For a Windows Store installation I think it’s at:
C:\Users\ {user-name} \AppData\Local\Packages\49800Krita_RANDOM STRING\LocalCacheRoamingkrita
(That was according to my interpretation of this section the manual:
Krita FAQ — Krita Manual 5.2.0 documentation )
Have you looked for these folders?
For importing ABR brushes, there are things you need to know, as explained here:
Essentially, you can only get the brush tip texture image, not the full brush behaviour.