Cannot log in to Plesk as a user to manage all the user's subscriptions:
The issue still persists even after resetting the password for this user in Domains > example.com > Web Hosting Access > System user
The system user is not allowed to log in to Plesk. Customers obtain their system users' access credentials from their service providers and use these credentials for connecting to the server through FTP, SSH, and so on. In addition, all operations with files and directories in Plesk are performed on behalf of system users. For example, when a customer adds a new file in File Manager, the subscription's system user becomes the owner of the file.
Only the following types of accounts are allowed to log in to Plesk: