[Derision]

That error is produced by this code:

#define _GASSIGN_TOLERANCE	1.0
	if(!(matches = mysql_num_rows(result)))	// try a fuzzy match if that didn't find it
	{
		mysql_free_result(result);
		RunQuery(
			query,
			MakeAnyLenString(
				&query,
				"SELECT id,x,y FROM spawn2 WHERE zone='%s' AND "
				"ABS( ABS(x) - ABS(%f) ) < %f AND "
				"ABS( ABS(y) - ABS(%f) ) < %f",
				zone->GetShortName(), x, _GASSIGN_TOLERANCE, y, _GASSIGN_TOLERANCE
			),
			errbuf,
			&result
		);
		safe_delete_array(query);
		fuzzy = 1;
		if(!(matches = mysql_num_rows(result)))
			mysql_free_result(result);
	}
	if(matches)
	{
		if(matches > 1)
		{
			client->Message(0, "ERROR: Unable to assign grid - multiple spawn2 rows match");

So it would seem you have two or more spawn2 entries for that zone that spawn in the same place, +/- 1 unit in the x and y direction. Try altering the spawn2 entry for the mob you are assigning the grid to so that it is at least 2 units away from the other entries in both the x and y direction.