Why does it give an error when parsing a deep page?

Question

Google

I parse flagma.ru. It became interesting to disassemble this site. Everything works well up to page 5 inclusive, then the following error occurs:
http://-//W3C//DTD%20XHTML%201.0%20Strict//EN
I.e. I make a request on page 6 and this comes out. What does this mean? Obviously, it's not in the code, because the rest of the pages up to 5 work.
also noticed in the code of each page there is something similar to an error in the doctype.
ps I'm using googlescript