Days ago, I got some error when I try to use PHP's CURL operate MailGun's email send API on my RMBP(Mac os X 10.9)

Internal SSL engine error encountered during the SSL handshake

SSL certificate problem: Couldn't understand the server certificate format

I didn't find any solution online, so the lasthing I did is restart. It's working after restart.

As I can remember, I did get PPTP problem weeks ago, also fixed by system restart, so Mac os x 10.9 is not stable than because.