After converting or making updates to a 4D database, the error "XML parsing error; invalid document structure" may occur during startup or connection to the database. Running the file through a XML validator may also show no errors, though the problem persists.

In this event, check all the plugins and components in the database to make sure that they are all updated for the version of 4D the database is running on. Temporarily removing some plugins and checking how the database runs without them will show which one will need to be maintained and updated.