This may be because a certificate required for this in your Apple developer account has expired. These usually have a duration of about one year.
30 days before expiration, you will usually receive an email from Apple about this. Insofar as we can determine this independently, we will remind you about it in the administration area via a clearly recognizable notice in the top left corner.
Should this be the case, we ask for a short contact via our live chat at the bottom right. For this, use the keyword "iOS Push certificate expired".