error oauth_problem=signature_invalid for POST

This is some Magento core/The client using to generate OAuth Signature issue.

The client generated signature and Magento generated signatures are mismatching that's the reason behind throwing this error.

For temporary testing, to works fine, you can just comment the Magento code that checks the signature and through errors, if it's mismatch

Here are the lines that check sinatures.

On file app/code/core/Mage/Oauth/Model/Server.php

Line number 547 to 549

if ($calculatedSign != $this->_protocolParams['oauth_signature']) {
     $this->_throwException('', self::ERR_SIGNATURE_INVALID);
}

Here you can see where $calculatedSign is the Magento calculated signature and $this->_protocolParams['oauth_signature'] is what we sending.

For testing purpose, you can comment this lines and work.