Saw this posted previously (http://wordpress.org/support/topic/plugin-makes-my-nav-menu-disappear#post-2512430) but the issue has not been resolved in this case.
It is conflicting with the dsIDXpress plugin (for MLS listings) and the issue only occurs on those MLS listing pages (dynamically generated, not actual pages).
The Title of the current page replaces all the Menu items for both Primary and Secondary Menus.
You can see the issue here: http://www.newventurerealestate.com/idx/mls-a1698958-199_ocean_ln_dr_905_key_biscayne_fl_33139
If the Post Types Order plugin is deactivated, the Menus function properly.
Any help is appreciated! Thanks!