From my tests with Sandboxie 5.16 (64-bit)
Everything will run in sandboxie (I used dragdrop to sandboxie).
Running Everything a second time within the sandbox will correctly bring up the first 'still running' instance.
Running ES within the sandbox will work correctly.
Running ES outside the sandbox will not communicate with the sandboxed Everything.
Running Everything outside the sandbox will not communicate with the sandboxed Everything and will start a new instance.
These are expected. Everything IPC / ES uses Windows Messaging, which would not be able to pass through sandboxie.
When running Everything outside the sandbox (with no Everything running inside the sandbox):
Running ES inside the sandbox will not communicate with Everything running outside the sandbox.
Running Everything inside the sandbox will not communicate with Everything running outside the sandbox.
Again, both are expected.
Running as administrator or with a standard user account made no difference.
Is there some setting I've overlooked that allows you to communicate with Everything inside the sandbox, from outside the sandbox?
edit: I can add Window Access to Everything.exe with EVERYTHING_TASKBAR_NOTIFICATION
This will allow ES outside sandboxie to send a query to Everything in sandboxie. However, while Everything receives this query, it is unable to reply.
edit2: If I add Window Access to Everything.exe with both EVERYTHING_TASKBAR_NOTIFICATION and IPCTEST unsandboxed ES can communicate with sandboxed Everything.exe