The Developer product is from an experience belonging to a group.
I asked my friend (he is the group owner) to try it and he got the same error I got AND a different one:
I do not have the web request details from the network tab for that one.
I used Chrome for the attempt to upload the image.
Upon the first time the request failed, I have switched to an incognito tab, ensuring that my extensions are not interfering with the request, which still did not work (same error)
Thanks for the reports. Our on-call engineering team has resolved the issue. If you continue to experience any problems, please let us know.
Important note: Auto-saving was experiencing issues, and there’s a chance your Studio progress was not saved—especially if you closed the place. However, if you remained in the Team Create session, your progress should start auto-saving again.
