we are running classifieds sites in many countries around the globe.
One of them seems to have incurreed a Google penalty (about a year
ago): www.locanto.ca

The PR of the tld is 0, all other sites we run have a PR of 3 / 4

We used to have a different domain tld for Canada before. Over a year
ago we changed the domain to .ca and the penalty hit the site. We
don't have paid links, we are not using any blackhat seo technics. The
sites' layout is the same across different countries.

The backlink structure for locanto.ca is not great but not worse than
most of other sites that we are running in other countries.

We looked at many different angles what could have caused the penalty
but couldn't find it.

Any help/suggestion is greatly appreciated.

PS: Google webmaster tools shows about 900 external links