Cart Variant suddenly not showing in cart or affecting default price

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I have a 2.0.18a store with several hundred items. The Cost Price for each item is $0.001, just enough so that it's non-zero. Each item is available in approximately nine different sizes, each listed as a Cart Variant with its own price.

This store has worked great for over a year now, then suddenly a few days ago the Cart Variants stopped affecting the Item Price while in the cart. The Cart Variant also no longer appears below the Product Name in the cart.

I'm not sure what the trigger was that broke the previously working system. To the best of my knowledge, there have not been any changes to the installed Joomla 2.5.8, or VM systems. There have been some minor server updates to the Ubuntu host, but I don't recall updating PHP or MySQL.

I tried updating the store to VM 2.0.20b, but that did not solve the problem.

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Not sure if this helps, but here's some more info.

I restored the entire server from a backup from 11 days ago, which is a good week before my last few working orders. Even that backup exhibits the same behavior of zero price, not respecting the Cart Variant.

This leads me to believe that it has something to do with the browser and not the server-side.

Still, not much of this makes any sense to me.

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More info:

I installed a copy of the site from a backup that is 2.5 months old, and everything seems to work correctly on that copy.

I still can't figure out what the trigger was that broke VM, however.