In Portal for ArcGIS, the /portaladmin/security/sslCertificates/importRootOrIntermediate operation fails to add a certificate to the keystore when the boundary value for the multipart/form-data request header contains equal signs ("=").
This issue could not be reproduced as long as the boundary specified in the 'Content-Type' header includes double-quotes around the boundary value. This is outlined in RFC 2046 Section 5.1.1.