Hi Jerome,
Were the other plugins you could install uploaded through the “upload” mechanism in your dashboard, like is being done with UpdraftPlus now? Or was it via the wordpress.org directory that’s built in?
Anyway… if the uploader in your dashboard isn’t working, then try the other method, which is to a) unpack the zip on your PC – you’ll then have a directory “updraftplus”. b) Upload that directory into wp-content/plugins via FTP, so that it becomes wp-content/plugins/updraftplus (remove the existing directory if you’ve still go the free version there).
Best wishes,
David