One way to tell if it's actually steam/SE saying you're not owning that skin at that time is to try and skin a block with the vanilla painting button.

Anyway, I've decided (a while ago, I've finished it just now =) ) to remove the ownership test and just use the same painting function that I used in the ownership test to just paint blocks, the problems with this is that client still has no idea what skins are owned so I'd have to show all of them, then also add a warning if it notices you skinning something and it not changing skin.

If that fails for some people then I can just safely blame keen and steam, because I'm just using their API directly to paint xD

I have almost all the DLCs, so not owning any skins cannot be the problem. ^^ Anyway I personally agree with the new approach, and perhaps in general it can be assumed that usually, people have most DLC.
So you can not see a possible issue in the log? Do you maybe have experience with load order related problems in the past?

EDIT: I just saw you already updated. I will update here if for some reason something of this problem persists, though I assume the best. :)