With the release of iOS 13.2 we started to receive support requests regarding an error message that says 'Corrupt Document - An unknown error occurred when loading the document'. Sorry, if this happens to you, too!
There are two scenarios where this might happen. Either when opening MindNode 6 for iOS itself or when opening a file. We are currently looking into these issues and try to figure out what's going on.
You can resolve the issue momentarily with a restart of the device.
Update: We were able to resolve one of the issues causing this. Please update to MindNode version 6.1.2