Hi Sebastian,
It sounds like you have UpdraftPlus files that are neither accessible to PHP, or to FTP, on your webserver. This isn’t something that can happen from inside WordPress – PHP programs (like WordPress and its plugins) don’t change file permissions (and usually can’t, even if they want to). In consequence, you’ll need to speak to your web hosting company, and ask them to 1) remove the folder wp-content/plugins/updraftplus for you, and to 2) advise you what ownership it had, such that it wasn’t possible for you to modify it either through WP or FTP (so that you can understand how it happened + how to prevent it).
Best wishes,
David