As reported here and elsewhere, the EU is considering the creation of a clearing house to keep trading with Iran despite US sanctions. Why is it not possible for the EU to conduct normal financial transactions with Iran without the involvement of US institutions?
Because the US has threatened to sanction European companies that trade with Iran. Most European companies that are interested in trading in Iran also trade with the US, so would be vulnerable to various kinds of sanctions.
In this article it says that the key point of Instex (the now established SPV) is to clear information about transactions inside and outside of Iran that are then offset instead of clearing funds between the counterparties directly. In this way the US sanctions would not be violated. That said, apparently the uptake is very low, also because non-Iranian counterparties want to avoid upsetting the US.