Are you using Firefox on a Mac? I’m curious if the result is the same on other browsers or older versions of the browser.
Was the action a file drag-and-drop operation or was it a copy-paste operation?
Does this discussion help at all? I had similar trouble recently because I had a mounted a host volume for the uploads but had not set the ownership to UID 10000. It was failing somehow even though the chmod permissions should have allowed any user to write to the directory. Setting the uid to 10000 solved it in my case, but I do not know if I was receiving the same error message you are. It’s worth a try though.