Yes, actually it looks like the problem exists only with Windows XP.
Beside that it would definetly be a good idea to upgrade to Win7/10, as it's from a security perspective more or less a nightmare to still browse around with Windows XP
I reproduced the issue yesterday in a virtual machine and it seems like the issue is the missing
SNI support in Windows XP.
Until we have reworked a couple of things we are actually forced to use SNI in our webserver due to the 2 domains we use.
So for now it looks like there are 2 (maybe 3) solutions:
1. Upgrade to a more recent and still supported version of windows (at least personally would definetly recommend that, Windows 10 seems to be the best choice at the moment. You can get it for around 20 Euros if you buy a windows 7 and do the free upgrade to windows 10).
2. Wait until we patched/reworked all the needed things to be able to switch of SNI (will probably take a couple of weeks).
3. We will try that one in the next days and if it works we will provide all the needed downloads: Install the needed scripts manually.
Actually this is one of the "never tested" things before the "go live" as noone of the admins seems to still use XP.
We are on it, but it takes some time