ASP.NET: Validation of Viewstate MAC failed

Hello,

I have a vb.net page which I copied as a template and changed all the page name references to itself to the new name. Now I'm getting this error:

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

And I'm seeing this message below, which means somewhere in the code the page is being redirected back to the original page ( /project_detail.aspx not  /project_template_detail.aspx) I made the copy from, but nowhere in the code that I can find is there a reference to that original page.

[ViewStateException: Invalid viewstate.
      Client IP: ::1
      Port: 55544
      Referer: http://localhost:54249/project_template_detail.aspx?iProjectID%3d1403%26iTaskID%3d0%26sAct%3dProjEd
      Path: /project_detail.aspx
      User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:46.0) Gecko/20100101 Firefox/46.0
      ViewState: /wEPDwUJNTE0MTUyNDY0D2QWBAIBD2QWAmYPFgIeCWlubmVyaHRtbAUGZGJUcmF4ZAIDD2QWJAIzD2QWBAIBDxYCHg

I tried setting  EnableEventValidation="false" and viewstateencryptionmode="Never" in the page directive, but didn't help.

Anyone have any idea what I'm missing?

Thanks!