The burden of proof shall be upon the party urging the invalidity, irregularity, or ineffectiveness of an agreement, deed, lease, or option described in Sections 26304 and 26305 or of any assessments, sales, or assessment deeds of which he complains to show the invalidity, irregularity, or ineffectiveness, and he shall be required to plead specifically the grounds upon which he urges the invalidity, irregularity, or ineffectiveness.