Parse server Unity - FCM "MismatchSenderId"

Question

I'm trying to send push notifications to Unity clients through Firebase Cloud Messaging with a self-hosted parse server. So far I succeeded in connecting to Firebase, but when I try to send a notification (through Cloud-code or Parse dashboard) I get the following message:

verbose: sending push to 1 installations
verb parse-server-push-adapter GCM sending to 1 device
verb parse-server-push-adapter GCM GCM Response: {
verb parse-server-push-adapter GCM     "multicast_id": 7055734461886567000,
verb parse-server-push-adapter GCM     "success": 0,
verb parse-server-push-adapter GCM     "failure": 1,
verb parse-server-push-adapter GCM     "canonical_ids": 0,
verb parse-server-push-adapter GCM     "results": [
verb parse-server-push-adapter GCM         {
verb parse-server-push-adapter GCM             "error": "MismatchSenderId"
verb parse-server-push-adapter GCM         }
verb parse-server-push-adapter GCM     ]
verb parse-server-push-adapter GCM }
verbose: sent push! 0 success, 1 failures

It seems like a clear message, but when I checked the Firebase console the sender_id seems to be right. I also checked the Installation collection in the Parse Dashboard, but the GCMSenderId field is the same as in server configuration. I also added the gcm_sender_id in the AndroidManifest.xml, but that didn't solve anything.

I'm really confused and hope someone can help me with solving this error.