I’m running High Sierra as well, and I’m running into the exact same problem. I tried diagnosing what I could of it, couldn’t find anything interesting in the Console except for a bunch of spew from syspolicyd (which isn’t really out of the ordinary).
4.4.2 works for you, but not 4.4.3? Maybe I’ll try to find a .dmg for 4.4.2 and see if it works for me.
I hope 4.4.3 does not actually require a newer version of macOS than the system requirements listed on krita.org (macOS >= 10.12) …
BTW – 2008 Mac Pro? That is ancient hardware, I’m glad it still serves you well.