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@demotruk could you elaborate ? I just did a search and cannot find any reference to these attacks ?

It has been mentioned by @ned in steemit.chat. The registration process is always being attacked by bots, who are trying to create hundreds or thousands of accounts at Steemit Inc's expense. Or they may just be engaged in a form of DOS attack.

You can see it in the graphs of new user accounts. Every few days they suddenly grind to a halt. Then Steemit fixes it for a while but it happens again when the attackers update their attack.