Have it solved. Had nothing to do with HestiaCP’s working, that was perfect (as always)
No, my ISP had to ‘update/upgrade’ the access to the internet, thereby blocking all outgoing traffic over port 21.
Changing the settings resolved connecting with FileZilla.
For future reference: My ISP is KPN (Netherlands), and the cable modem Security level (beveiligingniveau) had to be set to low (laag) to allow use of my own settings.