"Download course exercises" failed with error message on first attempt. Subsequent attempts fails silently

[motin]

Steps to reproduce
Install IntelliJ IDEA CE (2018.2), added the TMC plugin and logged in, chose the organization and course, then pressed "Download course exercises".

Expected result
The course exercises should start downloading into the folder for projects.

Actual result
A progress bar showed up, but an error message came up stating that the downloads failed (asked me to check the internet connection - despite being on a perfectly healthy and stable internet connection).
Nothing got downloaded into the Folder for projects.
Subsequent attempts fail silently after the progress bar has popped up for a few seconds.
Nothing gets logged in idea.log.

Notes
Tried shutting down IntelliJ, removing ~/Library/Application Support/IdeaIC2018.2 and starting from scratch, but the TMC-related settings are still the same as before, and the error remains.
After also removing ~/Library/Preferences/IdeaIC2018.2, the settings were reset, but after logging in and choosing the course etc the download fails silently.

[motin]

(Workaround for this issue is to use Netbeans with TMC to Download the course exercises, and then go back to IntelliJ to complete and submit the exercises)