On further inspection it seems your entire CA Chain is using SHA-1 certifaces. Your end client cert is SHA256. To get your SHA256 cert working after 1/1/2016 the StartCom Intermediate Certificate in the chain should be also at least SHA256, preferably also the root in the chain, which is also still a SHA1 signed cert.
Please consult your CA provider asap. This is really a big shame on your CA Provider that is still using SHA1.