I think I am beginning to understand the issue…
I just deleted the FTP Account of the tosy user and than also removed the /srv/jail/tosy_root directory.
So I guess, the user ideec also had an FTP user. But that FTP user was not removed when I run v-change-domain-owner, or when I deleted the user ideec from HestiaCP UI. That would explain the existence of the directory /srv/jail/ideec_root (of the now deleted user ideec) which points to the same directory as /srv/jail/tosy.
I suppose this means that the command v-change-domain-owner needs to be corrected in order to check for FTP user accounts and remove them from /srv/jail.
Now… As far as I remember, the directories in /srv/jail are symlinks (or something) bind mounts (thanks @sahsanu). How do I remove the obsolete ones (without deleting the actual data of course 
)?