Kindly ensure that you upload the same CSR which was downloaded via Hexnode -> Admin -> APNs. This error shall occur if by chance a wrong CSR file is uploaded instead of the one found in your Hexnode account.
This error could also occur when the downloaded file was opened and a change was accidentally affected on it prior to uploading it. Kindly ensure to keep the file un-edited while downloading and uploading to the APNs console.
Please have a look at this below help document for more info in the same.