If the issue is specific to one report, check if the template is attached to the program in xml publisher responsibililty
When submitting the program from Submit request window, is the layout picked up correctly and check the output is correctly displayed under layout in the same form.
Try to do a preview from xml publisher responsibility using the same xml data file and template.
Check if the xml file geenrated is openable from browser.
Issue is with all reports with XML output. Template correctly attached with reports and in browser also it's showing raw XML file only.
I have recently applied RPC 5 patches since then I'm facing this issue.
What are your OPP Memory values and temp space.
1) Ensure that OPP memory is set to 2GB
2) Temp has atleast 5-10G of free space. Temp location can be obtained from "Xml Publisher" responsibility -> Administration -> path