Hi,
If the credentials are known to work elsewhere, then:
1) Perhaps there is a firewall on the webserver that blocks some parts of the FTP connection
2) Try turning off SSL (if you are using Premium - in which case you shouldn't be in this forum (https://wordpress.org/support/topic/please-read-support-forum-guidelines?replies=1)) - in the 'expert' settings section of the UpdraftPlus settings page.
Best wishes,
David