macOS 10.11.6 Arduino app stops launching at "Initializing packages..."

[systembolaget]

Yesterday evening everything worked as it did for years. This morning, without a macOS update or installing other software, the Arduino app (13.02.2020) stops launching at "Initializing packages..." and the splash-screen self-closes.

What is this and what should one do?

[per1234]

Closing as duplicate of #10332.

Please see #10332 (comment) for an explanation of the problem and the fix.