karman after decoding and translating this seems to be the error message for "The avatar field is required"
If the error says "is required", it means Flarum probably isn't receiving the file at all, so it could be a client side issue, or something with the webserver/proxy before the request is handled to Flarum.
Does it happen on multiple different browsers? Does it happen with all files or only some file extensions or only one particular file?
If it always fails with any file and browser, maybe your web browser is somehow removing/ignoring the files that are sent with the requests? What kind of hosting are you using, and can you maybe contact their support?
