The serial number is not valid for this product

We currently have a site license for CS3, CS5 and CS6. CS3 is no logner in use, but we still use a mix of CS5 and CS6 apps.

Recently a massage would come up on random machines stating that the serial number is invalid.

I have tried retyping the number to no avail. I know for a fact that the serial number is correct, as we've been using it for years without any issues.

I've gone through the steps on the following page, but it made no difference.

https://helpx.adobe.com/creative-suite/kb/error-serial-number-valid-product.html

I have also tried a fresh installation of Windows 7. I manually installed CS3 and CS5, both serial numbers were accepted during the installation process, but then as soon as I tried running the software, CS3 reports connection failure and CS5 reports invalid serial numbers.

I also realised that I can no longer download updates for CS5 using Adobe Application Manager which I found a bit odd.

This makes me think that this is some sort of connectivity issue, resulting in both adobe activation and update servers to become inaccessible.

I have disabled the firewall, to rule out this possibility, but it made no difference.

Our ISP is adamant that they are not blocking anything. That leaves with me with a small number of other possibilities:

1. Adobe pulled the plug on older CS packages and servers responsible for activation and updates for these are no longer operational.

2. Our serial numbers have been blocked for some reason.

3. Adobe is having issues with update/activation servers.

Contacting Adobe regarding this is rather difficult, as both products are no longer supported and the only support option I have is chat. Unfortunately chat is either permanently closed, or at least it is every time I check, most likely due to me being in GMT time zone and these guys operating from within the US.

Anyway, I am running out of ideas and any help would be appreciated.