Hello, Simone.
At this point we are quite confident that this problem is a manifestation of two Microsoft bugs working together:
1. On Windows XP http.sys always negotiates client certs even when instructed not to. Since hardly anyone uses http.sys on XP, especially with SSL, and even more so with client certificates, nobody seems to have noticed this problem. Microsoft is clearly aware of the problem as this is not happening on Windows 7, for example.
2. When client cert is negotiated with the client (usually a browser), as always happens on XP with http.sys configured for SSL (see problem 1), server tries to help the client with presenting eligible client certs by sending it server's list of trusted root certs, which on XP contains about 370 certs (compare to barely 40 on Windows 7). This huge number of certs is above what SSL protocol allows, so most clients deem it SSL error and quit - thus the error you have reported. On Windows 2003 this can be worked around by changing registry settings, and we'll test and try to fix it. On Windows XP, unfortunately, registry fix does not work, so the only way to make this problem go away is to remove some trusted root certificates on Windows XP, which UWS installer can't be tasked with, as this can easily be considered as malicious.
Fixing either of these problem would be a big help, although for SSL to work well with client certs, both would have to be fixed. But if client certs are not involved, fixing either would do. Unfortunately, our experience of working with Microsoft tells us that these issues are very unlikely to be fixed, especially given that XP is circa 2001, and three generations removed from the latest version: Windows 8. We will file this problem with Microsoft support, but realistically, SSL support on XP out of the box is unlikely.
Best regards,
UltiDev Team.
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