Opt-out Mechanisms Stopped Working After Upgrade

I've upgraded from 1.8.3 to 1.10.1. The iframe-based opt-out mechanism, which used to work flawlessly, stopped working. Checking or unchecking the tickbox shown in the iframe causes the tickbox to immediately revert to its prior state (so neither can you opt-in nor can you opt-out).

To be precise, I have several sites running on the same server, in different domains. Piwik admin and UI is on its own domain. All sites, however, point their own location, via Apache config file Alias directive, to Piwik's physical directory (Alias /piwik /var/xxxx/piwik), so:

site1.com/piwik -> /var/xxxx/piwik
site2.com/piwik -> /var/xxxx/piwik, and so on.

The iframe is called from within each own site: src="http://site1.com/piwik/index.php?module=CoreAdminHome&action=optOut&language=en"

As far as I can see, Piwik is no longer able to manipulate the opt-out cookie, i.e. no cookie is being added or removed, for each site. However, it is able to manipulate the cookie, and the opt-out, for its own admin/UI site—however, that one is kept private.

I wonder what has changed since 1.8.3 with regards how the opt-out cookie is being generated and stored.

PS. I've just reported it as a bug http://dev.piwik.org/trac/ticket/3724