I would guess that you have turned on the 'XML strict headers' setting under Administration -> Server -> Debugging. That option is only useful to developers, do not turn in on in a production setting.
(If you cannot get into Moodle at all, go into your database and UPDATE mdl_config SET value=0 WHERE name='xmlstrictheaders';)
What is going on is that becuase an error message "<b>Notice</b>: Undefined property: ..." is being displayed, that is making the page invalid HTML. And the whole point of the 'XML strict headers' is to tell the web browser not to display invalid HTML, which is very good for developers trying to fix bugs, but not if you just want to run a site.
Hardware and performance
XML Error when logging to our development environment
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