The first paragraph on the nature of the concern in the ICIG letter to McGuire clarifies what the alleged violation of the law is:
Here, the Complainant's Letter alleged, among other things, that the
President of the United States, in a telephone call with Ukrainian
President Volodymyr Zelenskyy on July 25, 2019, “sought to pressure
the Ukrainian leader to take actions to help the President’s 2020
reelection bid.” U.S. laws and regulations prohibit a foreign
national, directly or indirectly, from making a contribution or
donation of money or other thing of value, or to make an express or
implied promise to make a contribution or donation, in connection with
a Federal, State, or local election. Similarly, U.S. laws and
regulations prohibit a person from soliciting, accepting, or receiving
such a contribution or donation from a foreign national, directly or
indirectly, in connection with a Federal, State, or local election.
Further, in the ICIG’s judgment, alleged conduct by a senior U.S.
public official to seek foreign assistance to interfere in or
influence a Federal election would constitute a “serious or flagrant
problem [or] abuse” under 50 U.S.C. § 3033(k)(5)(G)(i), which would
also potentially expose such a U.S. public official (or others acting
in concert with the U.S. public official) to serious national security
and counterintelligence risks with respect to foreign intelligence
services aware of such alleged conduct.
That is, it is alleged that it is a violation of federal election law for a foreign national to aid a US election (by providing information, which might be of value).
The underlying statute is ambiguous. One reading is that the term refers to a
deficiency relating to the funding, administration, or operation of an
intelligence activity within the responsibility and authority of the
Director of National Intelligence involving classified information
and also
A serious or flagrant problem, abuse, violation of law or Executive
order
The alternative is to take the entire list and limit the scope of the violations etc. to those
within the responsibility and authority of the
Director of National Intelligence involving classified information
The interpretive canon known as the "last antecedent rule" favor the narrowest scope possible the immiidately above phrase.
The law does not require a Supreme Court quality analysis of the underlying law: the proper interpretation of that statute is far from obvious, see here. Whether or not the last antecedent rule would be actually invoked in a final appeal is very hard to say, but generally the courts disfavor the supposition that any statute is ever written ambiguously (that still doesn't tell us what the scope of the last phrase is). The wording of the ICIG letter clearly indicates his interpretation of the scope of the DNI phrase, as not being limited to only intelligence activities within the responsibility and authority of the Director of National Intelligence involving classified information.
However, the ICIG letter also notes that
the Director of National Intelligence has responsibility and authority
pursuant to federal law and Executive Orders to administer and operate
programs and activities related to potential foreign interference in a
United States election
Additionally, Executive Order 13848, Imposing Certain Sanctions in the Event of Foreign Influence in a United States Election declares that
the ability of persons ... outside the United States to interfere in
or undermine public confidence in United States elections...
constitutes an unusual and extraordinary threat to the national
security and foreign policy of the United States
putting the combination of foreign + elections within the scope of the DNI. The ICIG letter also reasons that
alleged conduct by a senior U.S. public official to seek foreign
assistance to interfere in or influence a Federal election would
constitute a “serious or flagrant problem [or] abuse” under 50 U.S.C.
§ 3033(k)(5)(G)(i), which would also potentially expose such a U.S.
public official (or others acting in concert with the U.S. public
official) to serious national security and counterintelligence risks
with respect to foreign intelligence services aware of such alleged
conduct.