Problem/Error
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HTTP Error 500.0 - Internal Server Error Calling LoadLibraryEx on ISAPI filter "C:\Parallels\Plesk\isapi\isapirewrite4.dll" failed

This seems permissions issue with psacln & psaadm users.
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Solution
Error indicates that ISAPI could not write because of permissions issue. Many times because of plesk updates or changes in windows environment, permissions get reset. It could be easily fixed with following-

1.Open Internet Information Server (IIS) Manager
2.Locate ISAPI Filters
3.Right click on DLL file “aspnet_filter.dll” go to properties
4.Go to security tab
5. Make sure users psacln and psaadm have rights of ‘Read’ & ‘Read&Execute’
6.Check same rights with the file "%plesk_dir%\isapi\isapirewrite4.dll"

Solved!