On www.napitdirect.co.uk (running Magento 18.104.22.168 - which we are unable to update unfortunately) we are sometimes unable to remove products or even alter the quantity in the shopping cart. It's not a consistent issue so it's difficult to diagnose but does happen regularly.
The issue can occur after adding any product to the cart and results in the message "cannot remove the item" so my first thought was that it was related to the SUPEE 7405 patch issues (https://community.magento.com/t5/Security-Patches/SUPEE-7405-Cannot-remove-the-item-from-Cart/td-p/27126) but it doesn't seem to relate to form_key and it's not a consistent issue as indicated in everything else I can find (though happens regularly enough to be a problem!)
We've tried clearing local cache, web cache, tested server status, tested different devices and IPs, but there isn't an obvious cause or fix and I can't find anything about it online.
Anyone else experience anything like this? Any fixes or ideas?