I had this problem when I use Ajax updatePanel.
Most often this error appears when we use Response.Write to modify rendered output, I am not aware of this error with load balancing, but just check, may it will be the reson:

1) check if you have the code that calls Respose.Write in code behind(just search for it in solution)

2) check if you have tag in markup

3) if you found one of above, replace them with literals and assign text to that literals in OnPrerender event of the page, it should solve the issue.

If you dont have Response.Write calls in the code, try to look at this article to find more: