Normally, income is taxed at the time the claim is acquired, i.e. as soon as you are entitled to it. This also applies to maintenance payments, even if these are temporarily classified as loans, i.e. deferred.
However, there is one exception to this rule: if it is uncertain whether you will actually receive the money, you wait until the payment has been made before paying tax. In the case of regular maintenance payments, the moment the money actually arrives is therefore decisive for taxation. This applies in particular if there is no clear agreement on deferral of the amounts.