Error setting cookie after upgrade to 4.4.1

Checked in multiple browsers, private mode etc. Seems to only happen when the page with Matomo installed is IFRAMEd. Any ideas?

piwik.js:12 There was an error setting cookie _pk_ses.1228.158f. Please check domain and path.
ak @ piwik.js:12
df @ piwik.js:33
cd @ piwik.js:41
cr @ piwik.js:44
b1 @ piwik.js:45
(anonymous) @ piwik.js:61
ch @ piwik.js:50
P.trackPageView @ piwik.js:61
af @ piwik.js:12
ad @ piwik.js:66
addTracker @ piwik.js:68
(anonymous) @ piwik.js:71
(anonymous) @ piwik.js:71