A theme with the name “themename 1011” and version already exists on the server.

Problem

When changing the Site Template of a site the following error is shown in the browser:

A theme with the name “templatename 1011” and version already exists on the server.
at Microsoft.SharePoint.Library.SPRequestInternalClass.ApplyTheme(String bstrUrl, String pVal)
at Microsoft.SharePoint.Library.SPRequest.ApplyTheme(String bstrUrl, String pVal)

Troubleshoot issues with Windows SharePoint Services.

Solution

1. Open the site in SharePoint Designer
2. Delete the _themes folder
3. Perform an iisreset
4. Try again, and it just works

This tip comes from here. Thanks!