Selenium: Session externalKey not available - selenium

Question

The Selenium Grid Hub I am running displays this error message whenever a Robot Framework automation test (kicked off by a Jenkins job) requests a Chrome browser from the Hub:
Session [(null externalkey)] not available and is not among the last
1000 terminated sessions.
I've spent hours searching and trying proposed solutions with no luck.
Running a Linux server as a Selenium Grid Hub and a windows server as a Selenium Grid Node. Have tried each node with versions 3.0.1, 3.5.3, 3.13.0, and 3.141.59 of the selenium-server-standalone-<version>.jar files. A third server runs Jenkins.
The Hub appears to be config'd properly because this works fine:
http://10.1.10.231:4444/grid/console
I've also run the jar files at the command line with -debug at the end but no further information is displayed. Have also tried -Dselenium.LOGGER.level=WARNING on the "java side" of the command line with no further information displayed than using -debug.
We have another set of servers with the same config that runs just fine.
Anyone have an idea of what this message is indicating?
Nitty-gritty follows.
Grid Hub:
java -jar -Xms1024m -Xmx2048m selenium-server-standalone-3.141.59.jar -role hub -hubConfig ./hub-config.json
hub-config.json:
{
"port": 4444,
"newSessionWaitTimeout": -1,
"servlets" : [],
"withoutServlets": [],
"custom": {},
"capabilityMatcher": "org.openqa.grid.internal.utils.DefaultCapabilityMatcher",
"throwOnCapabilityNotPresent": true,
"cleanUpCycle": 5000,
"role": "hub",
"debug": false,
"browserTimeout": 360,
"timeout": 1800
}
Grid Node:
SET CHROMEDRIVER=.\bin\chromedriver_win32-v2.37\chromedriver.exe
SET IEDRIVER=.\bin\IEDriverServer.exe
SET FFDRIVER=.\bin\geckodriver-v0.11.1-win32\geckodriver.exe
START "SeleniumNode" java -Xms1024m -Xmx2048m ^
-Dwebdriver.gecko.driver=%FFDRIVER% ^
-Dwebdriver.ie.driver=%IEDRIVER% ^
-Dwebdriver.chrome.driver=%CHROMEDRIVER% -Dwebdriver.chrome.args="--disable-extensions" ^
-jar .\bin\selenium-server-standalone-3.141.59.jar -role node -nodeConfig .\node-config.json
node-config.json:
{
"capabilities":
[
{
"browserName": "firefox",
"maxInstances": 5,
"platform": "WINDOWS",
"seleniumProtocol": "WebDriver"
},
{
"browserName": "chrome",
"maxInstances": 5,
"platform": "WINDOWS",
"seleniumProtocol": "WebDriver"
},
{
"browserName": "internet explorer",
"maxInstances": 5,
"platform": "WINDOWS",
"seleniumProtocol": "WebDriver"
}
],
"proxy": "org.openqa.grid.selenium.proxy.DefaultRemoteProxy",
"maxSession": 5,
"port": 5555,
"register": true,
"registerCycle": 5000,
"hub": "http://10.1.10.231:4444",
"nodeStatusCheckTimeout": 5000,
"nodePolling": 5000,
"role": "node",
"unregisterIfStillDownAfter": 60000,
"downPollingLimit": 2,
"debug": false,
"servlets" : [],
"withoutServlets": [],
"custom": {},
"browserTimeout": 0,
"timeout": 1800,
"cleanUpCycle": 2000
}

Answer 1

Figured this one out! Hopefully it will help others who have reported this error message and not received a solution.
As is most often the case with an issue like this, especially when there's a known-working setup right beside the broken instance, it was an (embarrassingly) silly config issue. The Robot tests were pointing to this URL to get a browser:
http://10.1.10.75:4444/wd/hub/static/resource/hub.html
and it should have been pointing to this path:
http://10.1.10.75:4444/wd/hub
At least it was an easy fix.