Useful or not, from you.
electron-builder AppImage auto-updater 404 when publish generic provider url has port

<!-- Which version of electron-builder are you using? --> <!-- Please always try to use latest version before report. -->

  • Version: 4.1.2

<!-- Which version of electron-updater are you using (if applicable)? -->

<!-- What target are you building for? -->

  • Target: AppImage

<!-- Enter your issue details below this comment. --> This configuration

"publish": [
    {
      "provider": "generic",
      "url": "http://127.0.0.1:8007/downloads"
    }
  ],

Results in

16:37:52.992 › Cannot download differentially, fallback to full download: HttpError: 404 Not Found
Headers: {
  "cache-control": [
    "max-age=0, private, must-revalidate",
    "no-cache, private"
  ],
  "connection": [
    "keep-alive"
  ],
  "content-type": [
    "text/html; charset=UTF-8"
  ],
  "date": [
    "Tue, 27 Aug 2019 19:37:52 GMT"
  ],
  "server": [
    "nginx/1.17.1"
  ],
  "transfer-encoding": [
    "chunked"
  ],
  "x-powered-by": [
    "PHP/7.1.14"
  ]
}
    at createHttpError (/tmp/.mount_Nota VlrDAch/resources/app.asar/node_modules/builder-util-runtime/src/httpExecutor.ts:30:10)
    at checkIsRangesSupported (/tmp/.mount_Nota VlrDAch/resources/app.asar/node_modules/electron-updater/src/differentialDownloader/multipleRangeDownloader.ts:104:12)
    at ClientRequest.request.httpExecutor.createRequest.response (/tmp/.mount_Nota VlrDAch/resources/app.asar/node_modules/electron-updater/src/differentialDownloader/DifferentialDownloader.ts:250:14)
    at ClientRequest.emit (events.js:182:13)
    at URLRequest.ClientRequest.urlRequest.on (/tmp/.mount_Nota VlrDAch/resources/electron.asar/browser/api/net.js:207:12)
    at URLRequest.emit (events.js:182:13)
16:37:53.095 › Error: Error: Cannot download "http://127.0.0.1/downloads/Nota%20Varejo-1.0.1.AppImage", status 404: Not Found
    at ClientRequest.request.createRequest.response (/tmp/.mount_Nota VlrDAch/resources/app.asar/node_modules/builder-util-runtime/src/httpExecutor.ts:247:26)
    at ClientRequest.emit (events.js:182:13)
    at URLRequest.ClientRequest.urlRequest.on (/tmp/.mount_Nota VlrDAch/resources/electron.asar/browser/api/net.js:207:12)
    at URLRequest.emit (events.js:182:13)
(node:19118) UnhandledPromiseRejectionWarning: Error: Cannot download "http://127.0.0.1/downloads/Nota%20Varejo-1.0.1.AppImage", status 404: Not Found
    at ClientRequest.request.createRequest.response (/tmp/.mount_Nota VlrDAch/resources/app.asar/node_modules/builder-util-runtime/src/httpExecutor.ts:247:26)
    at ClientRequest.emit (events.js:182:13)
    at URLRequest.ClientRequest.urlRequest.on (/tmp/.mount_Nota VlrDAch/resources/electron.asar/browser/api/net.js:207:12)
    at URLRequest.emit (events.js:182:13)

Notes on http://127.0.0.1/downloads without por number <!-- If you want, you can donate to increase issue priority (https://www.electron.build/donate) -->

That's a useful answer
Without any help

This problem occurs when remote file does not exists, only the log is affected