Debugging Elastic Layering problems is described in the documentation here (scroll down to Debug an Elastic Layer):

https://docs.citrix.com/en-us/citrix-app-layering/4/layer/assign-elastic-layers.html

Get the ELM logs first. Then:

1.Collect the data from these logs in the end-user VM after producing the problem:

• Windows Application Event log: In the Windows Event Viewer under Windows Logs, export the Application event log as an EVTX file.
• Unidesk Layering Service log (ulayersvc.log): C:\ProgramData\Unidesk\Logs\ulayersvc.log
• Unidesk LayerInfo log (layerinfo.log): %LOCALAPPDATA%\Unidesk\Logs\layerinfo.log (note, this is in the current user profile)
• If this is a UPL deployment, also collect: C:\ProgramData\Citrix\Logs\upl-support.log

2.Collect the values of these Registry keys from the end-user VM:

• HKEY_LOCAL_MACHINE\SOFTWARE\Unidesk\ULayer\AssignmentFile
• HKEY_LOCAL_MACHINE\SOFTWARE\Unidesk\ULayer\RepositoryPath
• HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\UserInit

3.Collect the contents of the JSON files, including at least the Assignment (ElasticLayerAssignments.json) and Layers (Layers.json) files from the Repository Path.

Enabling Debug Logging in Ulayer

By default, ulayersvc.log only logs events INFO or higher.  You may determine that you want more detailed logging.  To turn on DEBUG logging, you need to edit a configuration file on a published image.

The image must have been published with elastic layering enabled or the file will not be present.

The setting must be available before Ulayer.exe starts, which normally means it must be present before the machine boots.  So you cannot set it from a GPO, for instance.

Edit C:\Program Files\Unidesk\Layering Services\ulayer.exe.config.  Scroll down to find this block in the Log4Net Appender section:

    <root>
      <level value="INFO"/>
      <appender-ref ref="LogFileAppender"/>
    </root>

Change INFO to DEBUG and save.