Can't add new menu items in specific theme only

Ok, I have a really strange issue. I’m using the Abundance theme on my site. I built the site locally and deployed it about a month ago on my live server. Since deploying, I am unable to add new items to my navigation menus when this theme is activated. If I switch to a different theme, I can add menu items again, but the Abundance theme has its own “mega-menu” system built in that I can’t use when other themes are activated.

I’ve searched high and low for how to fix this, and have come across many posts regarding changing Suhosin vars, changing memory alloted to PHP, etc. and none of them work. The theme developer insists it is a PHP configuration issue, but my host is at a loss. Obviously there isn’t a PHP issue if I can add menu items when another theme is activated. If I save the menus in that theme and switch back to Abundance, the menus still remain saved, but I can’t edit the mega menus without having Abundance activated. If I save the menu when Abundance is activated, it deletes my last two menu items and all the sub-menus below it. I counted, and it seems like there’s a limit of about 74 menu items in Abundance because if I remove other items from the menu, new menu items will save and appear. But once I hit that threshold, I can’t add any new items.

Does anyone have any idea what might be causing this and what I can do to fix it? Thanks so much!