Endless redirect loop on some subsites

On the latest release https://github.com/humanmade/Mercator/commit/306176a3b795220722a48920aadee550e73a1ccb - compared to what I had before I pulled (https://github.com/humanmade/Mercator/commit/0f07fc68ec88a1074704c250aad81fcb04e7bdb4), I get the following behaviour.

Two of my subsites go into an endless redirect loop between their proper www.domain.com address, and something like rootsite.com/wp-admin/ajax.php?page=mercator-sso (could be that or page=mercator-sso-something) with a few other (long) arguments in the query string. The other sites are fine.

My setup: I have several sub-sites on a multisite network. Each sub-site is accessed by its own domain; I have the sub-site's domain www.domain.com as the sub-site URL set in network site settings (info tab), and an alias for non-www domain (active) and domain.rootsite.com (active). The two sub-sites that have a problem have domains that are hosted by hosting provider 'B' (whereas the rest of my sites are hosted by provider 'A'). I have confirmed with domain provider B that there are no problems with the DNS or anything their side.

I have tried removing all cookies. I have tried using private browsing mode in safari.

As these are production sites, I had to use git to hard reset.
+1 vote for tagging stable releases please :) My clients are counting on it.