IIS ImageResizer getting 504 gateway timeout error after moving to another IIS Server

Question

now: we have asp.net project that use ImageResizer to crop images and It caches on Disk. below packages installed by Nuget:

ImageResizer.3.4.3
ImageResizer.Plugins.DiskCache.3.4.3

and below my Web.config resizer section:

  <resizer>
    <diagnostics enableFor="Localhost"/>
    <plugins>
      <add name="MyProject.ImageResizerLocalReader"/>
      <add name="DiskCache"/>
        <add name="DefaultSettings"/>
    </plugins>
    <sizelimits imageWidth="3200" imageHeight="3200" totalBehavior="ignorelimits"/>
    <diskCache dir="~/imagecache" autoClean="false" hashModifiedDate="true"/>
      <defaultsettings explicitSizeScaleMode="Both"/>
    <clientcache minutes="10080"/>
  </resizer>

it works properly without problem on production edge

goal: we want to transfer images and this project to another server (windows VM). configuration on both VMs are the same (IIS Version, Installed Softwares, Windows Revision, RAM and CPU Resources etc). even project works and it serve images properly when we manually test it (before moving it to production network)

problem: after changing VM network to production, server randomly respond with too high response time and it often responses 504 Gateway Timeout Error. but on current server all is OK! (even both VMs are the same)

Thanks for your help and suggestions.

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My custom plugin (MyProject.ImageResizerLocalReader) code goes in this gist: https://gist.github.com/sadeghmohebbi/53b0f367daa165756e084ff8a8ff9f23