@Julien2 Can you please confirm below things.Have you installed the AEM Forms Rights Management?Is this reoccurring error message coming in logs?

As of now It's not possible to customise the OOTB email template of AEM Forms Document Security. You can change the text/message body but can't customise it with scripts tags etc.

For AEM Forms as a Cloud Service local sdk, you need to use https://mvnrepository.com/artifact/com.adobe.aem/aem-forms-sdk-api sdk.

As of now It's not possible to customise the OOTB email template of AEM Forms Document Security. You can change the text/message body but can't customise it with scripts tags etc.