Thank you for contacting Sitefinity support.
Unfortunately this specified cryptographic algorithm (in this case SHA256) that Sitefinity use for encrypting is not supported in Windows XP and Windows Server 2003, although it is possible to overcome this by installing a hotfix. Look at this KB for more information
However, it is recommended to upgrade your server at least to Windows Server 2008 or newer to avoid such kind of problems.
I hope this information helps. Please let me know if I can assist you any further. Thank you in advance
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