MarketPress - Add to Cart just spins without completing


Doing my first build with MarketPress at

I've worked through a number of problems including white pages only (conflict with W3 Total Cache) then memory exhausted errors (conflict with JetPack), have turned off almost all my standard 'old faithful' plug-ins and now at least have a Store that loads and responds fast, except...

Adding either of the two listed products "Add to Cart" just spins without completing.

Couldn't find any other posters with this issue.