Unable to connect to Zapier

I have used Zapier successfully with Sendy for years. Lately none of my Zaps are working although they don't show errors in the Zapier interface. Their tech support sent me the log file of one of my Zaps I use to update Salesforce when a customer unsubscribes from emails etc.
This is the log.

"Credentials are censored and data may be formatted and/or truncated.

permalink: /messages/graylog_17476/86b72053-0869-11ed-bb80-926093b356af
timestamp: 2022-07-20T20:21:24.000Z

REQUEST

POST http://sendy.goldtouchinc.com/includes/helpers/integrations/zapier/triggers/hooks.php?api_key=:censored:20:85cf44fa28:&your_sendy_installation_url=http://sendy.goldtouchinc.com

Content-Type: application/x-www-form-urlencoded
Accept: application/json
user-agent: Zapier

url=https://zapier.com/hooks/standard/1729133/e85274ce263e46aa8d014342df7c33da/
event=new_user_unsubscribed
list=H8C763oNoZqXSVJkp0X6bxYw
brand=1
append_data=1

RESPONSE

400

connection: Upgrade, close
content-length: 42
content-type: text/html; charset=UTF-8
date: Wed, 20 Jul 2022 20:21:24 GMT
server: Apache/2.4.46 () OpenSSL/1.0.2k-fips PHP/5.4.16
upgrade: h2,h2c
x-powered-by: PHP/5.4.16

{
"message": "Unable to subscribe to hook."
}
"