"Oops! A URL in your Tweet appears to link to a page that has spammy or unsafe content."

Following their "Learn more" link takes you here: https://support.twitter.com/articles/90491-my-website-is-being-flagged-as-malware-or-spam

But Google Safe Browsing reports the destination as not suspicious, only Twitter are blocking it.

I've reported it via https://support.twitter.com/forms/spam but really do not believe the report will be looked at in a timely way and the time critical nature of the campaign means that I hope if this gets prominence on HN that someone at Twitter will look at it and ensure all URLs to that domain are white-listed (or at least removed from their current black-list).