Problem with Triggerdialog Plugin

3rd Party Addons
1

Mautic: 4.3.1
Triggerdialog: 2.0.0. (I think so)
PHP: 7.4.3

I try to send a letter via a Triggerdialog Campaign, but receive the error:
Letzter Ausführungs-Fehler: json_decode error: Syntax error
(Last execution-error: json_decode error: Syntax error)

I find this errormessage in the event list of every contact which I tried to send the letter to.

Any idea how to solve the problem?

(edit):
I found a log entry:

mautic.CRITICAL: Uncaught PHP Exception GuzzleHttp\Exception\InvalidArgumentException: “json_decode error: Syntax error” at [mautic-path/]vendor/guzzlehttp/guzzle/src/Utils.php line 275 {“exception”:"[object] (GuzzleHttp\Exception\InvalidArgumentException(code: 0): json_decode error: Syntax error at [mautic-path/]vendor/guzzlehttp/guzzle/src/Utils.php:275)"} {“hostname”:"[host]",“pid”:279266}

Best Regards
F. N.