FCM/APNS integration works fine for new tokens, but not when converting old APNS tokens?

Question

I have an FCM project sending push notifications to both iOS and Android users successfully. New iOS users subscribe and retrieve an FCM token.

I however also have a number of users on old iOS apps with direct APNS subscriptions and I tried converting one old APNS token to an FCM token and send a message using this via FCM.

I used the API described here: https://www.thepolyglotdeveloper.com/2017/06/apns-tokens-fcm-tokens-simple-http/

Sending a message using FCM to the converted token, I get the error: Auth error from APNS or Web Push Service. A stackoverflow article on this error suspects project setup issues, but can this even be the case if iOS users do get pushes with new FCM tokens?

Any hints on where this goes wrong and what else I could try out would be appreciated, I am very much in the dark here.

Answer 1

Found it myself eventually. I had used the wrong value for the "application" parameter in the conversion API call.