Upgraded angelic sickle sabre to angelic sickle shamshir using the exceptional set weapon cube recipe, and paladin can equip it but assassin can’t. All equip requirements are met on the assassin, even without wearing any gear or charms. Attempting to equip simply makes the “clanging” sound like it equipped, but with no error message and the item remains attached to the cursor.
Check your character level.
I actually can equip it after hitting level 28, even though the shamshir says level 23 required, and I was level 24 at the time. After some time the item tooltip also updated to say level 28, so I guess the bug was purely tooltip-based.