After upgrading from 1.5.4b to 1.6, no more data was being collected by the server due to DB schema changes. Unfortunately the patch provided in the dbupgrades directory is for a 1.4 to 1.6 upgrade.
Here is a patch (barely tested) for going from 1.5.4b to 1.6.
Here is a patch (barely tested) for going from 1.5.4b to 1.6.