Most probably you application pool runs uses ASP.NET 4.0, but the project targets lower version or you are running the project in the opposite configuration.
"Stack Trace: /r/nError: The server method 'GetContentItems' failed./r/nStatus Code: 404/r/nException Type: /r/nTimed Out: false"
Below are the reasons for this error.
1. Url Rewriting
2. Web service is blocked
3. You have changed the web service - passing not valid values as parameters.
5. You have images/documents that have some invalid metakey values.
The problem is that the web service cannot process its GetContentItems method ( Sitefinity\Admin\Services\ContentService.asmx
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