Google Adsense uses a lot of ad partners that generate ads- around 30,000+ of these partners. Unfortunately, sometimes these ad partners systems are not working properly and when they are called your browser would return a whole load of 404s or ERR_CONNECTION_REFUSED.

s0.2mdn.net is one that often returns problems; it is owned by Doubleclick which Google bought out recently.

When you notice this happening; you can login Adsense to block the offending partners; as of 2022; the interface was as follows, this may change quickly as Google likes to keep changing their interfaces to throw people off.

Block certain partners

To block 2mdn.net you can try to block all of the DoubleClick partners; I’ve already done that but after 2 hours of adding the blocking I’m still getting the entries on my site.