All acts, proceedings, conclusions, and findings of fact, including the levy of assessments or charges, by a board of a district concerning a distribution district therein shall be conclusive except in an action or proceeding instituted within six months after the acts, proceedings, conclusions, or findings were had or made.

An action to determine the validity of an assessment may be brought pursuant to Chapter 9 (commencing with Section 860) of Title 10 of Part 2 of the Code of Civil Procedure.