As stated in the title running the library with php 8.1 trigger an unexpected behavior with \GuzzleHttp\Psr7\Stream::getContents.
For instance in ContactApi::requestContactExportWithHttpInfo this code:

if ($returnType === '\SplFileObject') {
    $content = $responseBody; //stream goes to serializer
} else {
    **$content = $responseBody->getContents();**
    if ($returnType !== 'string') {
        $content = json_decode($content);
    }
}

return a $content = "".

I didn't go into details, but I found that might be a stream issue.
If I cast $responseBody into a string it works as expected since $content turn to be something like {"processId":90} thank to magic method GuzzleHttp\Psr7\Stream::__toString.

$content = (string) $responseBody;

Maybe it's a guzzle http issue, not a brevo php sdk issue.

Thanks in advance