The modifier input value for menu items is an int property, which is not compatible with the enum property used in node group socket definitions for menus. Wrapping a node group with a menu will try to assign the int value from the modifier directly to the enum, which expects a string identifier. To aquire the matching string identifier, use the UI properties in the modifier RNA, which contain the complete enum item definition. Pull Request: https://projects.blender.org/blender/blender/pulls/139074