Your code tries to open a file without specifying a path name.
In this scenario the resulting location of your file is the IIS process startup directory, not a folder inside you web site.
If you want to reach a file inside your web site folder you use Server.MapPath.
As explained in the docs, if you don't set any relative path then the file is assumed to be present in the same folder where is located your ASP page.
So, you could write
XMLObjectParser parseri = new XMLObjectParser(Server.MapPath("XML.xml"));
if your file should be located in the current folder relative to the calling page.
You can specify any folder inside the folder structure of your web site using something like this
XMLObjectParser parseri = new XMLObjectParser(Server.MapPath("/APP_DATA/XML.xml"));
More info on Server.MapPath here