Hey all, I was looking at the EU version of this thread (as people there are having the same issue)
someone suggested that an Avast antivirus update may be doing it, sure enough, I tried disabling my shields temporarily, and my screenshots are coming through again!
https://eu.forums.blizzard.com/en/wow/t/screen-capture-failed/219997/12
EDIT: I was able to “whitelist” WoW by going into Avast settings > General > Blocked & Allowed Apps > then manually adding World of Warcraft to the allowed ones!