Column not found: 1054 Unknown column 'sliders.thumbnail' in 'field list'

Error message

Column not found: 1054 Unknown column 'sliders.thumbnail' in 'field list'

Cause + Solution

The problem in your website is, that you probably updated by simply copying the files up your server, but we modified the database, and the database modification codes are only running with the installation code. So now you should log out from your admin backend, go to your database and into the core_resource table. In there you should find the nextend2_setup and smartslider3_setup values, and delete those rows. Refresh your admin login page two times (the first one will probably drop a php error, but the second refresh will make that disappear), login and it should be fine. Your sliders will stay.