When a QWebEngineDownloadItem changes its state to DownloadInterrupted, there's no way to figure out what error happened.

Chromium seems to provide that information, at least there's a list of possible values in chromium/content/public/browser/download_interrupt_reason_values.h.

I'd imagine an API with an error() method (returning an enum value) and an errorString() method (chromium seems to have std::string CONTENT_EXPORT DownloadInterruptReasonToString( DownloadInterruptReason error);.