Fatal error: Class 'N2PluginBase' not found

PHP error

Fatal error: Class 'N2PluginBase' not found 

Other possible error: Class ‘N2CacheImage’ not found

Solution

Follow the "alternative update" documentation's FTP method:

The problem is caused by that installing/updating your Smart Slider 3 didn't worked, some of the files are missing probably because of a server issue. A reinstallation (which is the same as the alternative update) would solve the problem!