[otrs] Error after upgrading from 2.2.6 to 2.3.1 (Got no UserID!)

Greetings, After following the upgrade procedure in the UPGRADING file (which seemed to run without a problem), I've hit a snag: If I now browser to http://xxxxx/otrs/index.pl I get the error: "Got no UserID! at /opt/otrs//Kernel/System/LinkObject.pm line 70." I've been scratching around, but nothing jumps out at me. Anyone have an idea what this is about? I'm trying the upgrade out on a test server and would like to resolve this before trying on the production machine. Thanks Henry