Symptoms
I have no issues using K2 workflow for SharePoint on the other site collections except for the root site collection. Here is the error: Error An error occurred during the processing of /K2WebDesigner.aspx. Code blocks are not allowed in this file. Troubleshoot issues with Microsoft SharePoint Foundation.
Diagnoses
The issue was caused by SharePoint Security Setting.
Resolution
Adding the following to each WFE web.config resolves the issue.
<PageParserPaths>
<PageParserPath VirtualPath="/K2WebDesigner.aspx" CompilationMode="Always" AllowServerSideScript="true" />
</PageParserPaths>