Well, I’m not sure which of the following steps had helped me.
I found out that the problem is “Principal Contact” email address. Apparently it’s a common problem and it’s described HERE .
I found This Topic and followed their advises.
First of all I reinstall OJS (which could have been the resolution), then I set SMPT to On by removing ; before some of the lines (see my post in the topic), and set “Principal Contact” to the e-mail which matches the domain (Journal settings >> Contact >> Principal Contact ).
Hope it will help.