Set max connections to 1 and try again. Seems like file-upload either has server issue or disabled resume/chunks for free user.
JDownloader is requesting a range/chunk but the server answers with wrong response code and is missing the content-range response header.
I will disable chunks/resume with next plugin update
This has nothing to do with updates as the latest change in plugin is from 18.07.2019
For the browser extension, please see
https://board.jdownloader.org/showthread.php?t=81740