I renamed my plugin folder - thus deactivating the plugin as the error was all I saw on the site index or in the admin.
I downloaded version 1.5.6 and installed it. It wouldn't install, the error was still there. But then the files were and the plugin could be deleted. I did that.
Reinstalled 1.5.6 and activated it. All was back to normal.
Until the upgrade is FIXED I won't be upgrading. >.<
guess you didn't debug it very well, or at all.