All acts, proceedings, conclusions, and findings of fact, including the levy of an assessment, by a board of a district concerning an improvement 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 or of warrants may be brought pursuant to Chapter 9 (commencing with Section 860) of Title 10 of Part 2 of the Code of Civil Procedure.