I cannot get rid of the Strict Standards Error on the from and back end.
Strict Standards: Non-static method K2ModelItemlist::getCategoryTree() should not be called statically, assuming $this from incompatible context in /mnt/data/vhosts/casite-482443.cloudaccess.net/httpdocs/components/com_roksprocket/lib/RokSprocket/Provider/K2/Filter.php on line 149
Any idea? I tried different way of getting rid of error messages and cannot find anything on this specific error.

Hello,
The notice (which will not affect your site's functionality) is being thrown from the roksprocket component not K2.

Thx. Rockettheme blamed the k2. ;)
Yes, it does not brake functionality, it is just eye sore. How to get rid of the message? Thx

The file which is raising the notice is /mnt/data/vhosts/casite-482443.cloudaccess.net/httpdocs/components/com_roksprocket/lib/RokSprocket/Provider/K2/Filter.php which is clearly not from K2.

Thx
I just turned off the error messages in general settings, following the Rockettheme suggestions.
It will be fine until website update.
Thx

A strict standards notice will not affect functionality. You are safe till then.