A mod on the subreddit said that the devs are updating the site for a "large release" and to "expect some delays, errors, and bugs. Please report any bugs you experience with our customer support", so maybe that has something to do with it? I'm having the same issue, only I don't get that popup it just hangs at "Loading art..." forever.

Contacting support seems largely useless, though.

I am having the same issue.

same thing with me

Support checked for me, apparently everything is ok now

I'm still waiting for a response from support

Mine is doing that too. Any help?

The same thing happened with one of my maps; I tried several tests, but the error persists. Did you manage to resolve it?