Yes, I am using the newest version of git tag 1.29.0.
I’ve found what the problem was, so if anyone encounters something similar, here is the thing:
Before running the Cardano node, I used the curl command to download four configuration files from the URLs provided in the guide. But as the URLs have moved to a new address, it downloaded HTML code saying that the URLs have changed instead of JSON file - so basically String instead of Object.
I used new URLs, and the problem was gone. And now everything is working fine.