Array to string conversion error

[Maaike]

I have no idea what happend. But the last couple of days I can not edit any off my products. There is a loading problem. So on advise I turned on the debugging mode. So It showed a bug. On every page I am opening. Makes working a bit hard. So I am hoping you smart people can help me.

Notice on line 2101 in classes/Tools.php 
[8] Array to string conversion.

On that line is this: 

curl_setopt($curl, CURLOPT_POSTFIELDS, $post_data);

I have no idea what is going wrong and why. 

It's part of this: 

if (function_exists('curl_init')) {
            Tools::refreshCACertFile();
            $curl = curl_init();

            curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
            curl_setopt($curl, CURLOPT_URL, $url);
            curl_setopt($curl, CURLOPT_CONNECTTIMEOUT, 5);
            curl_setopt($curl, CURLOPT_TIMEOUT, $curl_timeout);
            curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, true);
            curl_setopt($curl, CURLOPT_CAINFO, _PS_CACHE_CA_CERT_FILE_);
            if ($stream_context != null) {
                $opts = stream_context_get_options($stream_context);
                if (isset($opts['http']['method']) && Tools::strtolower($opts['http']['method']) == 'post') {
                    curl_setopt($curl, CURLOPT_POST, true);
                    if (isset($opts['http']['content'])) {
                        parse_str($opts['http']['content'], $post_data);
                        curl_setopt($curl, CURLOPT_POSTFIELDS, $post_data);
                    }
                }
            }
            $content = curl_exec($curl);
            curl_close($curl);
            return $content;
        } else {
            return false;
        
What is wrong her and why is it popping op now?