Omeka community member Iwe Muiser found a bug in the upgrade process from Omeka 2.0 to Omeka 2.1. Many thanks!
That bug is now fixed in Omeka 2.1.1 and is available for download. Omeka users should upgrade to the Omeka 2.1 series by moving directly to Omeka 2.1.1.
Omeka 2.1.1 also includes minor improvements and fixes to the Seasons and Berlin themes.