Missing resource folder

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.