Login error: access_denied due to Unexpected token { in JSON at position 19

I have an access_denied error that occurs only for some users in my web app when they try to login.
Using the auth0-js 9.3.3 plugin for a VueJS 2.0 SPA using the authorization extension.

I get the following response, with an unexpected token error. How can I debug these kind of errors?

{
“date”: “2018-06-03T11:15:15.478Z”,
“type”: “f”,
“description”: “Unexpected token { in JSON at position 19”,
“connection”: null,
“connection_id”: “”,
“client_id”: “1ySh5N0sOXxMkcAslnuhRfxO5BloY56t”,
“client_name”: “IRIS”,
“ip”: “80.57.245.139”,
“user_agent”: “Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.181 Safari/537.36”,
“details”: {
“body”: {
“wa”: “wsignin1.0”,
“wresult”: “eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJ1c2VyX2lkIjoiNWFlODM0ZTc5MjA4YjgwNThhNGEyMDFkIiwiZW1haWwiOiJtYXJnZXJ0aG8xQGdtYWlsLmNvbSIsImVtYWlsX3ZlcmlmaWVkIjp0cnVlLCJsYXN0X3Bhc3N3b3JkX3Jlc2V0IjoiMjAxOC0wNS0xMlQxNDo1Njo0My4xNzFaIiwic2lkIjoiVVZQSThRWG9RdHRlNzg4c2c0Yy14VHZqNnFRRHdQVHMiLCJpYXQiOjE1MjgwMjQ1MTMsImV4cCI6MTUyODAyNDU3MywiYXVkIjoidXJuOmF1dGgwOjUwMXN0OlVzZXJuYW1lLVBhc3N3b3JkLUF1dGhlbnRpY2F0aW9uIiwiaXNzIjoidXJuOmF1dGgwIn0.lnK7k568DtWiUUEQEqn1PIOAQGeGQ6kg2Y6cwZUyl655ae-9dA-uM4oijD3ByDwVBL8aqFxxAAZmdPOr8pSpehTgsI7WTYrZee1sT2i80zG2IaKb-0Ue8Yx_7aGNMzhXVZHdkdY13EL5gzNeV52IVlhQLmUtDL9C4LZqApjN7wk”,
“wctx”: "{"strategy":"auth0","auth0Client":"eyJuYW1lIjoibG9jay5qcyIsInZlcnNpb24iOiIxMC4xOC4wIiwibGliX3ZlcnNpb24iOiI4LjcuMCJ9","tenant":"501st","connection":"Username-Password-Authentication","client_id":"1ySh5N0sOXxMkcAslnuhRfxO5BloY56t","response_type":"token id_token","scope":"openid profile email groups permissions roles","protocol":"oauth2","redirect_uri":"http://localhost:8080/callback\“,\“state\”:\“s31LeZ-DQZLfAX10cTZ4AcoP9E7-nl-w\”,\“nonce\”:\“0j2CVd8Aogz2sgh8MaetsgEEq-uKo0sN\”,\“sid\”:\“UVPI8QXoQtte788sg4c-xTvj6qQDwPTs\”,\“audience\”:\“https://iris.501st.nl\”,\“realm\”:\“Username-Password-Authentication\”,\“session_user\”:\“5b13cdc121652a131b057eb4\”}”
},
“qs”: {},
“connection”: null,
“error”: {
“message”: “Unexpected token { in JSON at position 19”,
“oauthError”: “access_denied”,
“type”: “oauth-authorization”
}
},
“hostname”: “XXXX.eu.auth0.com”,
“user_id”: “auth0|5ae834e79208b8058a4a201d”,
“user_name”: “XXXXX@gmail.com”,
“log_id”: “90020180603111515478182853610644826347557693433397116978”
}

Sorry, there’s no actionable information here to help you. Can you post some code snippets on how are you using our libraries? Also, can you tell us how are you configuring the extension etc?