And of course I have this file with the path mentioned in error logs. Though, I've decided to google the problem and found out that deleting ./ from my swagger file in the following line - $ref: './extdef/PaidOrderNotification.yaml' solves the problem.
But not once and for all.
There's a case like this: GetProductsPricesResponse: type: array items: $ref: "./extdef/Price.yaml"
Seems like when the type is array the workaround mentioned above doesn't work. It works only if you 'hardcode' the Price object in yaml document without using a reference path but using it as a definition, which is not good for me, cause if I do it with every case like that within my default yaml file, it's gonna be enormous and will be hard to read.