Change domain ftp user password via 'admin > ftp user > select domain name' (for admin) or 'admin > clients > select one > ftp user > select domain name'. And then change their password.
Hmmm ... very strange. I did the same as you did:
Home (Admin) --> FTP Users --> Select domein name (mydomein.com) --> Update (change) Password
The same is a strange thing in the 'Action log' I do not see that you change the FTP password.
I'm the last one who did it, you can see here in the picture:
Pic. -->
https://prnt.sc/g9q4wjSo two things are strange for me:
1.) I did the same thing as you did for changing FTP password and it did not work.
2.) Why do not I see your FTP 'update the password' moves in 'Action log' ?
And there is a third thing, when I added a new domain (after your ftp fix) I did not need anything to touching about FTP. The FTP worked correctly from the first connection and password was by default (Kloxo-MR panel password).
Regards.