No action of any kind shall be instituted for the purpose of contesting or enjoining the collection of any special assessment, unless (a) a protest is filed at the hearing held to confirm the special assessment roll; and unless (b) such action shall be commenced within thirty (30) days after the confirmation of the roll. If the city attorney submits a written opinion finding said roll illegal, in whole or in part, the city council shall revoke its confirmation, correct the illegality, if possible, and reconfirm the same. Property which is not involved in the illegality shall not be assessed more than was imposed upon the original confirmation without further notice and hearing thereon.