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defect in the existence or quality of contractual assent
Explanation: This is the translation both my NO-EN legal dictionaries provides.
This explanation follows:
Tilblivelsesmangel should not be translated "defect in the formation of contract" since that expression in English refers to all the vitiating factors which make a contract invalid, including lack of capacity ("habilitetsmangel"), defects in the contents of the contract ("innholdsmangel") as well as defects in the existence or quality of contractual assent.