In Microsoft Dynamics NAV 2016 CU1, support for VS2015 Community Edition has been implemented.
However if you do a default install of VS2015CE, the RDLC editor is not included and your report will show up as XML instead of the visual designer.
To fix this, enable Microsoft SQL Server Data Tools as part of the installer.
And it works again.
Editors Note: On this website you will find a longer description of the issue.