Unable to build any WebGL projects on Linux

Whenever I try to build a project for the WebGL platform on Linux, the following error is thrown in the console:

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ArgumentException: The Assembly UnityEngine.WebGLModule is referenced by UnityEngine ('/home/xxx/Unity/Hub/Editor/2020.3.17f1/Editor/Data/PlaybackEngines/WebGLSupport/Managed/UnityEngine.dll'). But the dll is not allowed to be included or could not be found. UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List`1[T] alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary`2[TKey,TValue] cache, UnityEditor.BuildTarget target) (at /home/bokken/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:158) UnityEditor.AssemblyHelper.FindAssembliesReferencedBy (System.String[] paths, System.String[] foldersToSearch, UnityEditor.BuildTarget target) (at /home/bokken/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:198) UnityEngine.GUIUtility:processEvent(Int32, IntPtr, Boolean&) (at /home/bokken/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:189)

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The above just happened on a brand new project but affects all of my projects. Whenever I open the "Build settings" dialog box, select the output folder making sure that WebGL is the selected platform, and then click "Build", the error is thrown in the console but no visible error occurs; output folder is empty but a temporary folder created by Unity does contains a bunch of files, however no build to be seen.

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How can I go about diagnosing and hopefully fixing this showstopper of an error?

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Some notes that I think will be helpful in diagnosing this issue:

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(1) Using Unity 2020.3.17f1 engine on Linux (Ubuntu 20.04 LTS) with latest Mono stable version installed (6.12.0.122).

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(2) The referenced assembly should exist as per the following:

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$ ls -lah ~/Unity/Hub/Editor/2020.3.17f1/Editor/Data/PlaybackEngines/WebGLSupport/Managed/UnityEngine.dll -rw-r--r-- 1 xxx xxx 89,088 Sep 1 12:54 /home/xxx/Unity/Hub/Editor/2020.3.17f1/Editor/Data/PlaybackEngines/WebGLSupport/Managed/UnityEngine.dll

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(incidentally, how the heck do you get this interface to add spacing between paragraphs?)