I've had it happen in Firefox, Chrome and Explorer. Before and after clearing caches.

Last Java update was 22 days ago, error began occurring after the maintenance for Rainlily's Event's second Phase.

Finally got around to filling a ticket for this on Discord, they said they'll take a look into it on Monday (They don't work weekends).

Anyone else who's been having the problem should go and talk to Anchan or whoever's there and submit a report as well.