Or, “things that happen when you use single-user licensing on a render node” 😉

In this case, a Houdini command-line render with Arnold (HtoA) failed because the AdskLicensingAgent failed to start.

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AdskLicensingAgent
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This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: minimal, offscreen, webgl, windows.

---------------------------
OK  
---------------------------

After AdskLicensingAgent crashed, the render would continue, but license authorization would fail:

00:00:00   148MB         | authorizing with license manager: user ...
00:00:31   153MB WARNING | rendering with watermarks because of failed authorization:
00:00:31   153MB         |  [clm.v2] timeout before callback was called

Using Process Monitor, I saw (as expected) that the problem was that the AdskLicensingAgent was loading Qt platform plugins from Houdini:

The solution? Set QT_QPA_PLATFORM_PLUGIN_PATH to point to the AdskLicensingAgent\platforms folder.

set QT_QPA_PLATFORM_PLUGIN_PATH=C:\Program Files (x86)\Common Files\Autodesk Shared\AdskLicensing\Current\AdskLicensingAgent\platforms