Hello,

did you recently migrate your project from GCM to Firebase Cloud Messaging? If this is the case, the migration has probably changed your sender ID.

Since Server version 6.1 still uses GCM, you should ensure that you specify the sender ID you had before the migration. Using it everything should work correctly, at least until Google enforces the migration a year from now.

Let us know if this is of help.
Best regards,

Gianluca