If you receive the message “the given object id is not that of a menu item”, this seems to be an issue with the standard WordPress menu system (note that this is not related to UberMenu, this article is just here to help if you run into trouble with the WordPress core menu system). It seems that it is the result of some type of database corruption
There are two things you can do:
1. Try repairing your MySQL database.
A use has reported that working with the same issue here
2. Increase your PHP memory_limit, as suggested here
It is likely your server is hitting its configured memory limit and preventing the menu save from completing successfully, leaving you with a broken menu.
There is also the possibility that you have reached a Menu Item Limit, and that those solutions may help