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The mistake was quite stupid: The structure A is defined over some base ring. For checking equality of the algebraic structures I used:

def __eq__(self, right):
    try:
        return self.base_ring() == right.base_ring()
    except:
        return False

However, since QQ.base_ring() is QQ, this meant that A over QQ and QQ itself were considered equal. Hence, sage thought there is also a coercion from A to QQ (and not only from QQ to A). Therefore, for QQ(2)*A.an_element() sage tried to convert A.an_element() to a rational which failed. On the other hand, A.an_element()*QQ(2) worked because QQ(2) could indeed be converted to an element of A. My fix was to simply change the equality function to something like

def __eq__(self, right):
    if not isinstance(right, A) :
        return False
    try:
        return self.base_ring() == right.base_ring()
    except:
        return False