The error point to another plugin, outside of K2. Judging from the name, it's some plugin that creates "social bookmarking links" to popular social sites. It throws an error however because it expects Joomla! core articles. I'd recommend disabling the plugin and trying K2 with its own internal social features (which I'm sure you'll find adequate enough). Additionally you can notify the developer of the other plugin and request they add support for K2 as well.
Fotis / JoomlaWorks Support Team
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Please search the forum before posting a new topic :)
