I recently migrated a suite of websites from an end-of-life Windows 2008 Web server to a new VPS running Ubuntu and Plesk. Most of the changes involved replacing some minor use of ColdFusion with PHP equivalents in the website code and reapplying various DNS settings on the new server.
One sticking point was migrating pre-existing FTP users from the outgoing Windows FTP service to ProFTPD. ProFTPD does not support the use of special or uppercase characters within usernames, however the outgoing Windows FTP service did. There are no settings in Plesk to change these restrictions which stem from the ProFTPD service.
The solution, after much searching, is to set up user aliases. the following format allows having specified aliases for compliant usernames on ProFTPD. This will allow you to give access via usernames with special characters. This should be placed within your /etc/proftpd.conf file or, better still, in a separate .conf file within the /etc/proftpd.d folder where it should be automatically scanned and included.
UserAlias MixedCaseUsername lower-case-username UserAlias User@Domain.tld user-domain-tld