Certificate Login Failure on Windows
On Windows, accounts guarded with a user certificate fail to log into ISDS (or create a new account) if the certificate has not been stored as a personal certificate in the system certificate store.
Adding personal certificate (.pfx file?) into system store: Control Panel -> Internet Options -> Content -> Certificates -> Personal -> Import ...
Datovka internally decodes the certificate file using OpenSSL but libcurl appears to take the certificates from the certificate store rather than using the supplied data (file).