Error "500 Internal Server Error" or "No Valid Session Found" Running tabcmd From Multiple Command Prompt Windows

Issue

When you try to run tabcmd from a new command prompt window on a computer where tabcmd is already running in an existing window, the command fails and one of the following errors might occur:

500 Internal Server Error

or

No valid session found (errorcode=46)

Environment

Tableau Server
Tableau Online
Tabcmd

Resolution

Use one of the following workarounds:

Wait for the first script to finish, then run your next command in the same window.
Run tabcmd on a different computer.

Cause

It is not currently possible to run multiple instances of tabcmd from a single computer.

Additional Information

When more than one tabcmd session is running at a time, tabcmd logs will show something similar to the following:

[58384] DEBUG 2018-06-07 18:46:21,818: ====>> Starting Tabcmd 9.2 at Thu Jun 07 18:46:21 CDT 2018 <<====
[50096] DEBUG 2018-06-07 18:46:22,917: ====>> Starting Tabcmd 9.2 at Thu Jun 07 18:46:22 CDT 2018 <<====
[58964] DEBUG 2018-06-07 18:46:22,935: ====>> Starting Tabcmd 9.2 at Thu Jun 07 18:46:22 CDT 2018 <<====
and then:
[58384] ERROR 2018-06-07 18:53:37,193: 500 Internal Server Error
[50096] ERROR 2018-06-07 18:53:37,242: 500 Internal Server Error
[58964] ERROR 2018-06-07 18:56:03,677: 500 Internal Server Error

In the above example, 3 different tabcmd sessions were started at the same time, and all 3 failed with the same error.
The same issue will occur if sessions overlap. The first tabcmd session needs to finish completely before the second one can be run.