Error sending custom email

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Hi everybody, since yesterday I get an error when I create a custom email in action templates.

Error: Error sending mail: Index (zero based) must be greater than or equal to zero and less than the size of the argument list.

It happens when I use the action in a process AND when I hit the "Send test mail" button in the action itself.
Does anyone have a solution?

webcon version is 2022_1_3

Best regards, Lutz