This can happen for a variety of reasons, but all point back to an XML file being invalid. Some common causes:
- a file you have uploaded was uploaded corruptly
- a component/module/etc you are using has a bad XML file
- there’s some mischief going on…
It can be a little tricky to figure out exactly which file (the error message only shows line/column!), so I find it handy to go into the administration area. From there, go to ‘Extensions’ -> ‘Install/Uninstall’.
Now click through the ‘Components’, ‘Modules’, ‘Plugins’, ‘Languages’, ‘Templates’ tabs. Note that the error could be in an XML file for any of these – and if you’re noticing it on every page, then I would suspect a Language or Template is most likely.
If the same error comes up when you go to a tab, then one of the items in that tab has an invalid XML file. From there I’d try checking the files on your site vs. known good copies (filesize can be a good indication), and re-upload any corrupted files.