This error can come from anywhere, but surely not K2. If you template supports K2, it may have outdated template overrides for K2. In that case, check if there's a file "itemform.php" inside /templates/YOUR_TEMPLATE/html/com_k2/default/ or /templates/YOUR_TEMPLATE/html/com_k2/templates/default/ and delete it.
Fotis / JoomlaWorks Support Team
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Please search the forum before posting a new topic :)