Yesterday I installed Kr 4.4.8 on an iMac. I also uninstalled Kr and reinstalled it. In the process Krita’s thumbnail viewer stopped working and I started getting a “Krita quit unexpectedly” msg. I wasn’t bothered because Kr was functioning.
Today I upgraded the macOS from 10.15.7 Catalina to 11.6 Big Sur and in the process Krita’s thumbnail viewer reappeared and the quit msg disappeared.
Perhaps other Mac users in the forum will find this useful.