MarketPress and Domain Mapping Conflict

Hello,

When MarketPress is enabled on a subsite using theme GridMarket with a mapped domain, the ADD TO CART option does not work and the EMPTY CART option at the top of the theme home page does not work.

When the mapped domain on the broken site is removed, then ADD TO CART and EMPTY CART work as expected.

When I re-enable the mapped domain then they both cease working again.

The questions are:

Is it GridMarket generating the issue?
Is it MarketPress generating the issue?
Is it Domain Mapping generating the issue?

This is discussed on this thread, without resolution and developers have not yet been engaged on the thread.

https://premium.wpmudev.org/forums/topic/marketpress-add-to-cart-does-not-work#post-420659

Any help would be appreciated.

Thanks.

Phil D