After authenticating to my search head this morning, the message "lookup_expander: One or more column names in the input CSV were invalid" appeared at the top of the browser window.
The server is running Splunk 5.0.5 together with version 2.4.1 of the Enterprise Security app.
Any assistance with troubleshooting this message would be appreciated.
check your assets.csv and identities.csv files, one of them has probably been corrupted. More info here.