Hi Leo,
Looks like you’re hitting an error from the underlying Node.js framework that the VSTS task is based on. More information here: https://developer.ibm.com/answers/questions/26698/unable-to-verify-leaf-signature-when-calling-rest-apis-from-node-js/
FYI you can leverage comments on VSTS-147 for hints on passing extra certificates if needed (though again this is a pure Node.js consideration).