The parameter isostr should accept an ISO 8601 format, but doing this is not work :
<?php
$dateStart = date_create('first day of next month this year');
$isoStr = $dateStart ->format(DateTimeInterface::ISO8601);
$datePeriod = new DatePeriod("R10/{$isoStr}/P1M");
// Throw Exception : DatePeriod::__construct(): Unknown or bad format (R10/2020-02-01T22:52:26+0000Z/P1M)
// While doing it by this way works
$datePeriod = new DatePeriod($dateStart , \DateInterval::createFromDateString("+1 month"), 10);
?>