Undefined index: contact in …/integration/core.php

This is the response returned by the Mautic API library:

array(2) {
  ["errors"]=>
  array(1) {
    [0]=>
    array(2) {
      ["code"]=>
      int(404)
      ["message"]=>
      string(460) "The response has unexpected status code (404).

Response: {"errors":[{"message":"Looks like I encountered an error (error #404). If I do it again, please report me to the system administrator!","code":404,"type":null}],"error":{"message":"Looks like I encountered an error (error #404). If I do it again, please report me to the system administrator! (<code>error</code> is deprecated as of 2.6.0 and will be removed in 3.0. Use the <code>errors</code> array instead.)","code":404}}"
    }
  }
  ["error"]=>
  array(2) {
    ["code"]=>
    int(404)
    ["message"]=>
    string(460) "The response has unexpected status code (404).

Response: {"errors":[{"message":"Looks like I encountered an error (error #404). If I do it again, please report me to the system administrator!","code":404,"type":null}],"error":{"message":"Looks like I encountered an error (error #404). If I do it again, please report me to the system administrator! (<code>error</code> is deprecated as of 2.6.0 and will be removed in 3.0. Use the <code>errors</code> array instead.)","code":404}}"
  }
}

@aerendir Please look into this https://stackoverflow.com/questions/52398485/error-occur-in-send-email-to-segment-through-mautic-apirq=1

It is a Mautic Specific issue.

Please look into this for suggested solution: https://github.com/mautic/api-library/issues/158

In the end it turned out the problem was really simpler: I had to simply delete all the cache and let Mautic recreate it (https://github.com/mautic/mautic/issues/6102#issuecomment-415489226).

Thank you for your really fast response, anyway! ??

Great plugin!