PHP，Dropbox API，OAuth签名和非拉丁符号

I have the PHP application integrated with Dropbox API. It worked good but since some time ago it stopped to work with non-latin symbols. For example, if i try to create a dropbox folder with the API and use Cyrillic letters in folder name then API requests fail with error {"error": "Unauthorized"}

I have the PHP function

function createOAuthSignature($apiurl, $params, $oauth_secret, $oauth_token_secret, $method='GET'){
    $urlencoded_apiurl = urlencode($apiurl);
    $urlencoded_params = urlencode($params);

    $basestring = $method .'&'.$urlencoded_apiurl.'&'.$urlencoded_params;
    $oauth_signature = base64_encode(hash_hmac('sha1', $basestring, $oauth_secret.'&'.$oauth_token_secret, true));

    $oauth_signature = urlencode($oauth_signature);

    return $oauth_signature;
}

And the calling code is

    $fullname = $parent . $foldername;
    $fullnameEnc = rawurlencode($fullname);
    $apiurl = $this->api_url .'fileops/create_folder/';

    $params  = 'oauth_consumer_key='.$this->oauth_consumer_key
        .'&oauth_nonce='. $this->createNonce()
        .'&oauth_signature_method=HMAC-SHA1'
        .'&oauth_timestamp='. time()
        .'&oauth_token='.$oauth_token
        .'&oauth_version=1.0'
        .'&path='. $fullnameEnc
        .'&root=dropbox';

    $oauth_signature = $this->createOAuthSignature($apiurl, $params, $this->oauth_secret, $oauth_token_secret);

    $params .= '&oauth_signature='.$oauth_signature;
    $action = $apiurl .'?'. $params;
    $s = $this->curl->get($action);

What can be the problem there? I presume it is related to signature generating. But what is wrong there? This code worked fine just couple weeks ago.

Thanks

As Greg mentioned in a comment above, we're investigating, but if you want to switch to PLAINTEXT signing, it's quite easy. Change createOAuthSignature to just do this:

function createOAuthSignature($apiurl, $params, $oauth_secret, $oauth_token_secret, $method='GET'){
    return urlencode($oauth_secret.'&'.$oauth_token_secret);
}

And in the calling code, change the signature method to PLAINTEXT:

$params = ...
    .'&oauth_signature_method=PLAINTEXT'
    ...