Piwik 2.1 - Error: Error unserializing the following response from (no replies)

I updated from 1.12 to 2.1.
When I run archive.php file, this process show :

Error: Error unserializing the following response from http://mydomain/index.php?module=API&method=VisitsSummary.getVisits&idSite=3&period=year&date=last2&format=php&token_auth=xyz&trigger=archivephp: INFO VisitsSummary[2014-04-21 13:04:26] [29b25] Purging temporary archives: skipped.
a:2:{i:2013;i:0;i:2014;d:8659888;}

what happen?

I need to resolve this problem.

Regards.
Mariano