Consider this scenario. You have SharePoint with SQL Reporting Services (SSRS) running in SharePoint Integrated mode. You want to use a SharePoint list as a data source for a report.

In the SSRS Data Connection you enter the URL and credentials for the SharePoint site but when you test the connection the following error appears:

The given key was not present in the dictionary

After a bit of trial and error I discovered that this error occurs if the URL entered isn’t the default URL for the web application.

To resolve the issue, go into Central Admin and check the Alternative Access mappings on the SharePoint site used in the data source. Check that you are using the default URL.

Similar but unrelated issue

Note that there is a similar issue that occurs when added managed accounts in Central Admin. These issues are not related. Further details of that issue can be found here: https://support.microsoft.com/en-nz/help/2463865/sharepoint-2010-error-message-the-given-key-was-not-present-in-the-dic