All debts, contracts and obligations heretofore made or incurred by or in favor of any sewer district, all bonds, warrants, or other obligations issued by such districts, any connection or service charges made by such districts, any and all assessments heretofore levied in any utility local improvement districts of any sewer districts, and all other things and proceedings relating thereto done or taken by such sewer districts or by their respective officers are hereby declared to be legal and valid and of full force and effect from the date thereof: PROVIDED, That nothing in this section shall apply to ultra vires acts or acts of fraud committed by the officers or agents of said district.