by Brett Andrew 14th April 2021

During a routine Kentico 12 to Kentico 13 upgrade, I encountered this issue.

Could not load file or assembly 'CMS.OutputFilter' or one of its dependencies. The system cannot find the file specified.

So this is actually expected as K13  upgrade notes specify:

• CMS.PortalEngine.dll and CMS.OutputFilter.dll merged into CMS.PortalEngine.Web.UI.dll
  
  This stumped me for a good few minutes, so putting this out there to help others.

In our situation the upgrade failed as we had too many settings in the web.config and the upgrade process failed to update the web.config for us and alerted to do it manually.... This specific settings is this:

BEFORE

<add name="XHtmlModule" type="CMS.OutputFilter.OutputFilterModule, CMS.OutputFilter" />

AFTER

<add name="XHtmlModule" type="CMS.OutputFilter.OutputFilterModule, CMS.PortalEngine.Web.UI" />

You should use the web.config.new file the upgrade created automatically and move YOUR sites changes into it, this will also ensure that any other changes required in the web.config are made and there are quite a few!

This also fixed another issue where the pages were showing an error Server error in"/" Application : Unknown server tag 'cms:CMSForm' which was being thrown by something else in the web.config.

Good luck!

Brett

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