I do not care what the wiki says because none of it worked.
I use LibreElec on my Raspberry Pi. I enabled SSH and I also have a password.
I connect to port 22 and my LibreElec IP address.
I tried "libreelec", "le", and my SSH password as the password when it prompted me.
As a result, I got "Access denied".
Help... 
root/libreelec are the default credentials but the first-run wizard will prompt you to change them. Make sure you didn't enable "disable password auth" in LE settings as this will require you to use SSH key authentication to login. If this is enabled you'll still receive a password prompt but no password will be accepted.
The two remaining reasons that I see SSH issues from are:
a) Users formatting /storage as FAT32, which ensures incorrect SSH key permissions and the daemon will fail to start.
b) User error .. fat fingers, typo, etc.
You can also try the web ssh addon from the repo, just install and start it.
Then you have a terminal at you browser
And just as a double check you did use root as username didn't you??
Is it possible to access LE with user root/password and alternatively with ssh keypair? And if yes - where to store ".ssh/authorized_keys" file?
Yes, just put it in .ssh/authorized_keys (full path /storage/.ssh/authorized_keys)
This works OOTB if you enabled ssh in the first run wizard (or later in LE settings).
so long,
Hias
where is disable password auth?
only see in the services panel
ssh enable
password disable
where is disable password auth?
only see in the services panel
ssh enable
password disable
Err, what you think "password disable" does?
password disable
Enable it, and use default password libreelec.
If you enable "password disable" passwords don't work, so do the reverse and they should work (again).
My misunderstanding of GUI message. 
