Hi Bob,
I don’t think this is a compatibility issue but rather an issue with your server limits.
I am not sure what is the default limit for max_input_vars in your server maybe the settings I gave is bit too high
that it will raise security concerns.
If for example the default limit is set max_input_vars 1000 and can accommodate only 50 menu items then maybe they can set it a little bit higher like max_input_vars 1500 to accommodate 70 menu items as oppose to limit I gave above which is max_input_vars 8000.
Can you ask your hosting company what is the default/current limit then ask them to make it a bit higher then test adding a menu item if it works.
Hope that helps.