Fixing “Calling LoadLibraryEx on ISAPI filter v4.0.30319 aspnet_filter.dll failed”
Calling LoadLibraryEx on ISAPI filter "C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll" failed

I ran into this adding a new site to my 64 bit machine, and because I haven’t had my morning coffee, I forgot to set “enable 32 bit applications” in the app pool.

If your code is built for 32 bit only (maybe you use mixed mode dll’s somewhere or call into native and can’t be 64 bit for whatever reason), make sure the app pool of your application is set to 32 bit mode. Otherwise, you get the very descriptive error shown above.



  1. frinkfree

    Although your solution fixes the immediate problem, I think the root cause of the error is the extraneous backslash near the end of the ISAPI filter entry “C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll”. Changing this value to “C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_filter.dll” will fix the error reported by IIS.

  2. Rickkee

    What causes that extra \\ to appear. Just default load, did not edit it, but with .net update, the \\ appears and wacked the site. Thanks for the answer, it worked beautiful after 14 hours of troubleshooting, should have come here first!

  3. Chris

    Many thanks – fixed my problem – but last week the site worked fine. Any idea why (.NET update?) this would break without changing any settings?

  4. kapil Dev

    HTTP Error 500.0 – Internal Server Error
    Calling LoadLibraryEx on ISAPI filter “C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll” failed

    Thanks to solve this issues
    multipal slash issues
    Framework64\\v4.0.30319\aspnet_filter.dll To Framework64\v4.0.30319\aspnet_filter.dll

    Hi All,

    I got this error, before you do any above stuff, just try below

    1. Click on your computer icon in IIS

    2. On right side, you see ISAPI filters – Click

    3. Pick up the Filter name ASP.Net_4.0_64bit and its path MIGHT HAVE AN EXTRA \ IN THE PATH,



    4. restart IIS – should resolve the error.

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