XHTML markup is incorrect near placeholder "header". For more information about this issue refer to Sitecore Knowledge base article https://kb.sitecore.net/articles/365101.
I know this error message shows when there some XHTML validation problem. But In our case its not related to XHTML validation.
The weird thing here is that if I change my placeholder key "header" (i.e. - "Header" or "header1") and update the presentation details, it resolve the problem. Don't konw what is problem with "header" placeholder key. here the code we are using on layout -
We are using Sitecore version - 8.2 update 2