while working on Jenkins' seed job, some changes I introduced didn't get reverted even after running the seed job again from the master branch. This is why seed job is now failing. More details here  and here .
Since I can operate only with jobs phrase-triggered from GitHub's PR, I think there's nothing more I can do than I already tried. In my opinion, fixing this issue requires an aid of a person with some greater access to Jenkins. Can someone help with that?
Sorry for the inconvenience - I didn't expect that such situation can occur. "job_seed_standalone" works fine and it can be used instead (until the issue is fixed).