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If your machines are all the same vendor, it really would be worth checking for a newer network driver (not necessarily one from the vendor which often are not updated). I had a similar problem ages ago where a certain brand of PC in my network would not browse unless the shield was off. Upgrading the network driver fixed it. I think it was a Broadcom network chip if I recall.

I have made a habit of keeping a copy of old installers (offline installer, not the small web one). Might be worth downgrading back to an older version you know works, at least you will not have to disable the component and can keep some strength in your protection.

Support should be able to supply you a copy of a previous release for your site. At least they used to be able to.

Wish you luck!