If you are pointing to the Sandbox server, recently there were some updates made that would cause that specific error.
Below is the information and link from the PayPal Merchant Technical Support Microsite on the SHA-256 Upgrade:
Support SHA-256. PayPal is upgrading SSL certificates on all Live and Sandbox endpoints from SHA-1 to the stronger and more robust
SHA-256 algorithm. You will need to update your integration to support
certificates using SHA-256.
Discontinue use of the VeriSign G2 Root Certificate. In accordance with industry standards, PayPal will no longer honor secure
connections that require the VeriSign G2 Root Certificate for trust
validation. Only secure connection requests that are expecting our
certificate/trust chain to be signed by the G5 Root Certificate will
result in successful secure connections.
Directly from the PayPal MicroSite:
SSL Certificate Upgrade
On January 19-20, 2016 The Sandbox endpoints will be upgraded to new
SHA-256, 2048-bit certificates:
api.sandbox.paypal.com
api-3t.sandbox.paypal.com
api-aa.sandbox.paypal.com
api-aa-3t.sandbox.paypal.com
svcs.sandbox.paypal.com
pointofsale.sandbox.paypal.com
ipnpb.sandbox.paypal.com
www.sandbox.paypal.com (for IPN)
Here is the link to the PayPal User Guide with detailed instructions on changing your Certificate to a G5 Root Certificate.