IntelliJ IDEA starts failed with "EOFException: Expected 6 bytes, got 0"

Problem

IntelliJ IDEA starts failed with the below exception:

Cannot connect to already running IDE instance.
EOFException: Expected 6 bytes, got 0

Cause

When IntelliJ IDEA starts, it checks if any IntelliJ IDEA instances are running because only one instance running per IDE config/cache folder is supported. If there is another IntelliJ IDEA instance running (e.g., frozen or failed to shut down in the previous session without any UI showing), and the current instance fails to connect to that instance, this fatal exception may occur.

Solutions

We have improved the error message in future versions to advise users on how to resolve this issue.

For now, open the task manager (e.g., Activity Monitor on macOS), kill all running IntelliJ IDEA processes, and start IntelliJ IDEA again. If the problem persists or you want to investigate further, please contact IntellIJ support by submitting a request or creating an issue here with IDE log folder zipped and a thread dump of from another running IntelliJ IDEA instance via jstack section here to investigate why it failed to shut down.