Try the following things:
- Make sure BizTalk360 IIS Application/Virtual directory is using the correct Application pool.
- If the problem still exists, Restart the server
- If the problem still exists, Delete the existing application pool, create a new application pool with correct identity and configure BizTalk360 with the new application pool.
If this error appears on an XP machine after a new install, simply uninstalling BizTalk360, log-off, login back and re-installing BizTalk360 should fix the issue.