Error 500 after PrestaShop upgrade from 1.5 to 1.6

Upgrading a PrestaShop 1.5 to 1.6 resulted in Error 500 when visiting the shop frontend. During the upgrade I had selected to disable all non native modules, replace theme with default theme and updating all emails.

I tried every trick in  the book, like cleaning the cache, disable all caching and compression, etc but it didn’t help.

The problem was solved like this:

• Go to Preferences -> Themes
• Make sure the default-bootstrap theme is selected
• Click on the Advanced options button for the theme
• It turned out the setting for Folder was not set to any folder (no radio button selected). Select default-bootstrap folder and Save

This solved the problem for me.