On line 45 you have %s two times in your select statement, but only pass one replacement value. Same thing on line 49.
45 should probably be:
$rsNext = mysql_query(sprintf("SELECT * FROM productos WHERE prodID = %s AND prodID > %s ORDER BY prodID ASC LIMIT 0, 1", GetSQLValueString($colname_rsProductos, "int"), GetSQLValueString($colname_rsProductos, "int")));
and 49:
$rsPrev = mysql_query(sprintf("SSELECT * FROM productos WHERE prodID = %s AND prodID < %s ORDER BY prodID DESC LIMIT 0, 1", GetSQLValueString($colname_rsProductos, "int"), GetSQLValueString($colname_rsProductos, "int")));