I decided to figure out how to recover data on a disk if the system does not start. It's better to do it before it happens;)
Since there is no definitive correct answer for Linux Mint 18.3 KDE anywhere, I had to find a solution myself.
Good topic for other OS like "Ubuntu": https://askubuntu.com/questions/238047/how-do-i-mount-an-encrypted-home-directory-on-another-ubuntu-machine
sudo ecryptfs-manager and exit (select 4 => Enter) (see bug 455709 mount: mount(2) failed: No such file or directory)
/home/UserName directory by checking every device in Dolphin. In my case, it is located on a separate volume (But in your case, most likely this path will be inside the home folder like: /media/mint/1234567-1233-1233-32423-1a95b2c5dc68/home/UserName)
Alt + . for show hidden files
Go to .ecryptfs folder inside and try to find .Private folder (in every new Folder you have to "Show hidden files" by pressing Alt + . !!!)
As final your .Private folder:
F4 for opening console hereecryptfs-recover-private ./.Private (Passphrase is your login PC Password !!! )
/tmp/ecryptfs.JSiaYsbz directory using cd
p.s. Use the flag --rw (in step 9) to mount the encrypted filesystem as read and write (read only as default!)