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

No irregularity or omission not affecting the substantial rights of the landowners within a district shall affect the validity of any act done or proceeding taken in relation to the district.