Backup via FTPS (explicit) fails with Curl error: Login denied

Dear colleagues,

I experienced an issue using an external backup repository on Plesk 12.0.18 machines using ftps. The ftp server uses ftps (explicit) connections.
Plesk 11.5 works fine (and ordinary ftp client like Filezilla as well), Plesk 12 throws the following error when trying to add the server via the web interface:

Unencrypted connections will work. Plesk 11.5 machines (with the same settings) will work.

I've analyzed the problem and...

Backup via FTPS (explicit) fails with Curl error: Login denied