I get the following error message each time I choose add item:

1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')}' at line 1 SQL=SELECT `catid` FROM m4py5_k2_additional_categories WHERE `itemID` = {intval()}

What to do?

Hello,

This issue is not related to K2.
This is caused by the K2 additional categories extension.

You need to address this issue to the extension's developer.