I'd speculate that there's probably some syntactically important HTML character that needs to be escaped or balanced, but if you're unable to find the issue I think you're going to need to zip up your report template and mail it to the helpful humans at
omniplan@omnigroup.com.
Steps for doing so:
- From OmniPlan's Help menu, select "Send Feedback…"
If prompted you need not include a file.
- Now open up OmniPlan -> Preferences and go to the HTML tab
- Select your template
- Use the context menu (right click or click the gear button) to "Reveal in Finder"
- Drag the whole folder into the mail message
- Include a brief description of the issue
-Tom