"Upload failed... undefined" - resolved spontaneously by waiting

I'm experiencing an intermittent (but irritating) problem with OctoPrint. Frequently, when I slice a model into gcode and try to upload it, I get this error message:

Upload Failed
More: undefined

In these cases, OctoPrint doesn't even bother trying to start uploading - it's an instantaneous answer. Also, nothing - absolutely nothing - appears in the log.

If I try again - even repeatedly - I get the same message. If I wait several minutes and try again, the upload of the exact same file succeeds without a hitch.

OctoPrint otherwise appears to be fine.

What is the problem?

Failed uploads of sliced files that are successfully uploadable a few minutes later.

What did you already try to solve it?

Uploading repeatedly. Using different slicers. Rebooting OctoPrint and the RPi.

Logs (octoprint.log, serial.log or output on terminal tab, browser error console ...)

I've checked ~/.octoprint/octoprint.log. It contains no indication of a failed upload or a failure of any kind. When the file uploads successfully, it logs the upload.

Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware, browser, operating system, ...)

Prusa i3 mk3. Has happened intermittently with several firmware revisions; just occurred with firmware version 3.7.2 (latest atm).

Raspberry Pi 3B+. running Raspbian Lite Stretch (Linux 4.14.79-v7+).

OctoPrint 1.3.11. Vanilla install with only installed-by-default plugins. No special configuration.

Files created on a Mac (macOS Mojave 10.14.6) via Tinkercad. gcode files sliced with Simplify3D 4.1.1. Uploaded to Raspberry Pi via WiFi. WiFi connection doesn't seem to be a problem: when these errors occur, I'm watching OctoPrint tracking the extruder process live.)

The Get Help template suggests that additional information such as the browser you're using might be helpful. Since the browser is half of the upload side of things, this is significant.

Also, try again in Safe Mode. If it works then one of the third-party plugins may be interfering.