A false Logical DRC error can be printed when a PAD symbol has an IOBDELAY=NONE property and the input path has all of its loads trimmed, so it is loadless.
This is an issue in the connectivity checking for Logical DRC.
This error can be avoided in several ways:
For general information about setting ISE software environment variables, see (Xilinx Answer 11630).
This issue has been fixed in ISE Design Suite 14.7.