issue creating sign request using template - error 400 Signer role must match the signer or signer group of the template.

We've created sign requests through the Box Sign API for a while now using templates to pre-fill data fields.
Since a short time we can't create the requests any more - instead we get an error message concerning the signer role. The weird thing is, that this also is thrown for templates that previously worked fine for us.

Was there a change in requirements for the payload?

Here's the error json, that we receive:

{"result":{"status":400,"message":"Bad request","help_url":"https://developer.box.com/guides/api-calls/permissions-and-errors/common-errors/","request_id":"0d4d574a4283afb29508121379dde3f07","type":"error","code":"bad_request","context_info":{"errors":[{"name":"role","message":"Signer role must match the signer or signer group of the template.","reason":"invalid_parameter"}]}}}

And here the payload we're using to call the sign request create API:

{"template_id":"b1f7cdf7-fb1e-4624-8a0a-02dd1f67f59c","name":"No 15 - Contrat Intelligence Relationnelle®","are_reminders_enabled":true,"email_message":"Salut Marie,\n\nci-joint le contrat de test pour la formation #104.\nTu peux vérifier si c'est bon comme ça stp? J'ai rajouté le RIB comme piece supplémentaire qui n'est pas à signer...\n\nBonne soirée \nJan ","email_subject":"Test pour Marie","external_id":"H2441","prefill_tags":[{"document_tag_id":"nomStagiaire","text_value":"Monsieur Dupont"},{"document_tag_id":"profStagiaire","text_value":"Psychopraticien"},{"document_tag_id":"dateNaissance","text_value":"10.10.1980"},{"document_tag_id":"entrStagiaire","text_value":""},{"document_tag_id":"adrStagiaire","text_value":"3 rue Principale \n12345 VILLE"},{"document_tag_id":"titreFormation","text_value":"Formation au modèle de psychothérapie Intelligence Relationnelle® – Formation de base #15"},{"document_tag_id":"datesFormation","text_value":"Session 1 : Du 13 au 17 Mai 2019\nSession 2 : Du 21 au 25 Oktober 2019\nSession 3 : Du 10 au 14 Februar 2020\n\t\t\t\t."},{"document_tag_id":"horairesFormation","text_value":"\n\t\t\t\t."},{"document_tag_id":"lieuFormation","text_value":null},{"document_tag_id":"coutTotal","text_value":"1.234 €, TVA non applicable (Exonération de TVA – Art.259B du CGI) "},{"document_tag_id":"formateur","text_value":"\n\t\t\t\t."},{"document_tag_id":"acompte","text_value":"123 € valant réservation"},{"document_tag_id":"solde","text_value":"456 €"},{"document_tag_id":"echeances","text_value":"jusqu'au 13/05/19 : 123 €\njusqu'au 21/10/19 : 123 €\njusqu'au 10/02/20 : 123 €\n\t\t\t\t."},{"document_tag_id":"dateSignatureSTF","text_value":"17/12/2024\n\t\t\t\t."}],"signers":[{"email":"example@yahoo.de"}],"parent_folder":{"type":"folder","id":"283643977746"}}

Update: It seems that the root cause of the issue was a stricter handling of roles that were created in the template.
We're using Word-templates with placeholders to create our templates. When loading the Word-document into the template, automatically two recipients (destinataires) are created - and both are assigned the role "signer". Since the fields are structured in a way that the first recipient (who is in fact the sender) doesn't need to sign anything, we now need to manually change the role to "final copy reader". After doing this the signature requests are created like before using the API-calls.