Beginning with version 7.1i, the I/O placer creates internal macros to better handle the automatic placement of LVDS pairs. This new macro causes problems if the designer LOCs the Master IOB to the Slave site and vice versa. This practice is unusual but valid.
"ERROR:Place:604 - Placer was unable to create LVDS pair for component clk_p due to the following reason: ERROR:Place - The following components are part of this structure: IOB clk_p IOB clk_n The reason for this issue is the following: All of the logic associated with this structure is locked and the relative placement of the logic violates the structure. The problem was found between the relative placement of IOB clk_p at site IOB_X0Y79 and IOB clk_n at site IOB_X0Y78."
Meanwhile, a tactical patch is available to fix this. The fix allows LOC constraints to override the macro creation under environment variable control.