Symptoms

On site collections that are not root site, the K2 for SharePoint workflow designer screen (K2WebDesigner.aspx) errors out with this error:

Server Error in '/' Application.

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The resource cannot be found. Description: HTTP 404.

The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly.

Requested URL: /sites/sSite Name]/K2WebDesigner.aspx

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Version Information: Microsoft .NET Framework Version:2.0.50727.5485 ASP.NET Version:2.0.50727.5491
 

Diagnoses

The issue was caused by custom Branding that is applied to the site collections.
 

Resolution

There are 2 options.

1.Use OOB branding for the Site collection.

2. Add Minimal.master from clean site to Master Page Gallery. Then add the following section in node in SharePoint web.config

<PageParserPath VirtualPath="/sites/*" CompilationMode="Always" AllowServerSideScript="true" IncludeSubFolders="true" />