PHP Fatal error: Class 'OC_Cache_File' not found

[syndrael]

Hello here's my error in the log file.
PHP Fatal error: Class 'OC_Cache_File' not found in /var/www/owncloud/apps2/files_tree/ajax/explore.php on line 31
OC version 6.0.3
Files Tree version 0.5
Thanks for your help.
Best regards