Hi, thanks for the follow up.
It sounds like this may be a bug. Would you mind taking a look at the issues page here: https://github.com/elastic/kibana/issues and see if there are any open issues related to this?
I have found one which may be relevant: https://github.com/elastic/kibana/issues/74769 which also links to another discuss issue: Kibana Alert and Action "Error: error validating action params"
If those issues are not related, please feel free to file a new issue in the issue tracker. If it is possible, please enable verbose logging in the Kibana server and attach as many logs as you can to the issue.
Thanks!