Hi, in back end I get the same error trying access categories as in front end, trying to access product:

There has been an error processing your request

Exception printing is disabled by default for security reasons.

Error log record number: 266342629561
Als my hostign provider told me to tell you: I have also found the error below in the php_errorlog in the folder ~/public_html/dev.test/:

Code:
[17-May-2015 17:35:41 UTC] PHP Fatal error: Call to undefined method Mage_Catalog_Helper_Category_Flat::isAvailable() in /home/buenamuj/public_html/dev.test/app/code/local/Sns/Ostore/Block/Navigation.php on line 28

can we fix this please????