Upon trying to upload a media based thumbnail to an experience event under the Configure Event page > Event Details > Media, the site returns Response from https://apis.roblox.com/assets/user-auth/v1/assets returned an error code 400 and enters an infinite loading, also locking out the ability to update the event and save other changes or when creating a new event.
This has been reproduced with multiple thumbnails, trying different file names assuming it could’ve been a false flag moderation issue, and trying different file types, resolutions, and sizes.
ROBLOX has been having intermittent connection issues the last 12-ish hours, I recommend waiting little bit longer and attempt your upload again.
For situational awareness, Error 400 means Bad Request.
Also this seems to be 400 bad request. Could you take a look at the inspection to check the Response payload that contains the error message, then we can suggest, thx!
We might locate the issue. Your event name is too long to be applied as the thumbnail title. Can you try to rename your event first (length < 50 characters) then try to upload the thumbnails again?