I encounter the problem of error 500 when I tried to save the menu after added a new item.
I understand this is an old issue and several methods on how to resolve it was tried but still unable to save the menu.
So, I created a topic in this section is because I want to suggest, but not ask for help.
I suggest wordpress developer to look for any possibility to make the custom menu to how the widget work? When we add the new widget, it automatically save when we dragged the item into the widget section. I think menu also use the similar method and may be this can resolve the irritated error 500 problem ?
When saving menu, there are more than 500 items running in my mysql although I've only not more than 15 items in my menu, and half of them are custom link.
Hope wordpress developer can consider my suggestion, but if you'd already tried, just ignore my suggestion.
Thank you and have a productive day.