When creating an I/O Planning project in Vivado 2013.2 and selecting "Migrate to RTL" from the Vivado flow navigator, after I hit ok I run into the following error:
ERROR: [Common 17-70] Application Exception: ProjUtils::setConstrsTypeOrError: Illegal CONSTRS_TYPE: 'CT_UNKNOWN'
This error occurs because Vivado is incorrectly trying to write to a .ucf file.
To work around the issue, manually save your I/O planning constraints (Ctrl + S) to an .xdc file. After this has been done, the "Migrate to RTL" should work without error.
This issue is scheduled to be resolved in the 2013.3 release of the Xilinx Design Tools.