"Unable to add item" and "that item is not available" error for no reason

On one of our purchasing pages, all of the items are giving the following pop up error message:

"Unable to add item" and "that item is not available"

The inventory on all items is unlimited. No one can add the items to cart. Need fixed ASAP.

I'm having the same problem.

Same here! Haven't changed anything in months and the error just started on its' own. Losing potential orders :((

One of my clients is running into this issue with a few of his products. Has anyone else found a solution?

Same things was happening to me, but I resolved it:

The store/shop page the products are linked to what disabled. Once I enabled that page, the products could be added to cart and purchased. 

It happens if the Products page is disabled.