Probably a mod item.
Viewing post in Ero Dungeons [Old Version] comments
Can confirm it's not just "mod" items . I've not installed ANY mods, only Patreon release versions, but had the same "placeholder item" in my armor outfit inventory.
I couldn't recall offhand what was missing, but some armor from prior version appears to have been removed or changed. Something between Patreon 2.7.5 and 2.8 (in my case, specifically 2.8.1hotfix2)