Already discussed about by players on US forums, posting it for visibility here as well.
Demon hunters cannot transform leather boots from Sepulcher of the First Ones into their respective tier boots (Mercurial Punisher’s Boots) - the game does not recognize the resulting item, tries to retrieve item information forever without an end. The demon hunter boots also do not drop in the raid, ergo this bug makes demon hunters unable to complete the Mythic variant of their tier set (possibly any variant of the set).
Some players managed to work around the bug by swapping specs (specifically Devourer), however for me personally I cannot transform Moccasins of Dreadful Absolution (Mythic) via catalyst at all, in any spec.
Edit: Workaround has been found. Swap loot spec from specific spec, to “Current”, then swap to Devourer spec - it “fixes” the bug for the time being and I confirmed it has worked for me and various other players.