I am back, with probably the simplest problem with the simplest solution on earth.
Here, add this to the Page tag in your aspx page.
By the way the problem occurs in .aspx pages using ASP.NET 1.1 and ASP.NET 2.0. and when you insert some characters into text fields on the page that could be interpreted as special HTML characters and confuse the parser.
Any alternative asked Eralper, well.. I need to dig deeper and come back later