The homestead of a married person cannot be conveyed or encumbered unless the instrument by which it is conveyed or encumbered is executed and acknowledged by both husband and wife, except as otherwise hereinafter provided. If either the husband or wife is incompetent the homestead may be encumbered for the purpose of paying or refinancing any valid existing lien for which the homestead might be sold upon order of sale or execution, or for the purpose of paying taxes, provided the instrument by which it is to be encumbered is executed and acknowledged by the competent spouse and the interest of the incompetent person is encumbered in the manner and form provided for encumbering the interest of an incompetent person in other real property. Any claim of invalidity of a deed of conveyance of homestead property because of failure to comply with the provisions of this section must be asserted within the time provided in sections 76-288 to 76-298.