Typically caused by your VPN or other security device - Please try after disconnecting.
Your Centercode implementation's files are hosted on Amazon Web Services / S3. Attempting to access/download/upload files from Centercode redirects to S3 to verify access. S3 then provides the incoming request token to your computer to grant access to download / upload the file.
If the files you're attempting to upload are a common format and you're experiencing the "Upload Error Occurred" message, this typically indicates that your VPN or other security device is rejecting the request for violating a VPN or infrastructure rule. This is often resolved by disconnecting your VPN and re-uploading your files.
To be able to attach files while on your VPN or security device, you'll need to reach out to your company's IT to apply any changes to your VPN. This is unfortunately not something Centercode can directly help with because the upload request is being rejected before reaching Centercode's infrastructure.