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Interesting, have you seen someone elaborating on a potential reason? It would be totally understandable for me if a site starts to produce errors due to overload, when there are maybe 200 (usually consumer sized) servers for 200,000+ users and only 20 of those servers handle the majority of the load - at least that is how I understand the network up to now...

I saw a user provided the explanation that it could be a block that was not witnessed properly and it seemed to be happening more often since HF 19.