Cannot upload TLS cert, Error: "SSL PEM Server Certificate file upgrade through HTTP is failed"

We are trying to upload a custom TLS certificate using the Webinterface

Maintenance - Upgrade - HTTP File Upgrade - SSL Server Certificate PEM File

but the Upload fails with the following error message

SSL PEM Server Certificate file upgrade through HTTP is failed

There wasn't any problem uploading the Trusted Root Certificate File and the DH Strong Encryption Parameter File

Maintenance - Upgrade - HTTP File Upgrade - SSL Trusted Root Certificate PEM File
Maintenance - Upgrade - HTTP File Upgrade - SSL DH Strong Encryption Parameter PEM File

The same error message is displayed, if we are trying to download the certificate from a TFTP Server

 

Security - Access - HTTPS - Certificate Download - SSL Server Certificate PEM File

We have already followed the steps detailed in the previous discussion

M4300-28G (GSM4328S) HTTPS Admin "ERR_SSL_PROTOCOL_ERROR"


So the SSL Server Certificate PEM File contains the private key, followed by the server certificate, followed by the intermediate ca certificate.
We generated a SSL Server Certificate PEM File by using the Webinterface

Security - Access - HTTPS - Certificate Management - Generate Certificates

and tried to upload the previously served certificate, but it fails with the same error message.

the device runs firmware version 12.0.7.10

Has anyone experienced such an issue before?
Is there any information regarding the required format of the pem file?
Any help would be appreciated.