Main Menue: I can not add more items to the main menue

Hallo,
I create a website using the enfold theme. At the moment, the main menue comprises 71 pages. If i now try to extend the menue, the next item is converted to a mega menue item after saving the menue. It is also moved from the second level to the top level. The checkmark for the position in theme (checkmarked was Enfold Child Main Menu) also got lost. According to the info.php the max_input_vars = 5000, so that the restriction of this value shoudn’t be responsible for this behavior.
I would be glad if you would have other ideas for solving this problem.
Kind regards and thanks in advance!