An error occurred. Your account may be over its quota or you attempted to
upload a folder.
The error is very obvious. It means the account is over the quota. But what if it isn’t? This error is generic, cpanel throws this, whenever it fails to upload the file, regardless of what error it returns. There is a possibility that your IDS (Intrusion detection system) is discarding the upload, so double checking the IDS log should help you to conclude that. But what if, that is also not the case?
Ok, that can actually still happen. It happens when the customer uses cloudflare and uses cloudflare to login to the cpanel using cpanel proxy and then use it to upload the file. Cloudflare sees the upload going through web and blocks it. So, just double check the domain he uses to login to the cpanel, and check whether it uses some kind of 3rd party web application firewall loaded application or not like Cloudflare. If it does, that could be the case!