Trying to install krita v 4.2.7.1 on win10 x64 and keep getting the error can't write to Qt5Concurent.dll. Tried running as administrator -- not sure what else to try.

I've no idea what the underlying reason for this is but as a workround, possibly a permanent one, can you try the Windows Portable 64-bit zip version?

It's in a list under 'All Download Versions' on the main website. Just Extract it in your downloads folder then go into the resulting folder and run the krita.lnk link.

Whether this works for you or not may give a clue as to what may be going wrong for you. (I always use the portable zip version on Windows.)

I appears I had to restart my computer before doing the install. I've run into this with some Affinity software: try to install; it fails; restart computer and install and it works.

That's probably because there was an unkillable Krita process hanging.