Symptom:
Action fails when mediatype is webhook: "Sending failed: SyntaxError: invalid json"

Cause:
The json made by Zabbix to call third party API is truncated becasue of a limit in Zabbix DB table [ALERTS], column: _PARAMETERS _

How to reproduce:
Use

based on text items in trigger description to allow the resulting trigger description to exceed the maximum size.

Example of truncated json:

Zabbix should truncate the field "trigger_description" of teh json instead of truncating the json itself.
When the media type is e-mail instead of webhook, the problem exists too (column "Message" is limited too), butthe difference is that the e-mail, even truncated, is sent anyway.
This problem is important because the API Calls fail, which means nobody get informed about certain events/incidents.

Workaround: monitor the events/alerts and detect failed alerts.