Steps to reproduce:

1. Create Slack media with instructions from https://www.zabbix.com/integrations/slack
2. Problem message sent, but reported as non-sent because of  error: SyntaxError: invalid json (at offset 1)
3. Resolve messages is not sent at all, error - channel_not_found

Result:
        Messages in Slack is not working as expected
Expected:
       Messages sends to Slack without errors, resolve and update works too

Comment:

 ** After some investigations i've found the reason.
Part of script from tutorial tries to get permalink for Slack message to attach it as tag to Problem in Zabbix:

function getPermalink(channelId, messageTimestamp) {

 var req = new CurlHttpRequest();
 req.AddHeader('Content-Type: application/x-www-form-urlencoded; charset=utf-8');
 req.AddHeader('Authorization: Bearer ' + params.bot_token);

 var resp = JSON.parse(req.Get('\{0}?Ftoken=\{1}&channel=\{2}&message_ts=\{3}'.format(
 Slack.getPermalink,
 params.bot_token,
 channelId,
 messageTimestamp
 )););

 if (req.Status != 200 && !resp.ok) {
  throw resp.error;
 }

 return resp.permalink;
}

CurlHTTPRequest method is not working properly with urlencoded URI, in result it fails to get good answer from server and get plain-text HTML instead of JSON, so script falls with error.