There could be any number of things wrong with the XML file itself (corrupted, hierarchy starts and then stops abruptly because of a bad export process, etc.)
1. Try to first validate the XML file. If it's invalid, then OpenRefine as a tool for conversion to CSV will not work using our XML importer.
2. Try to verify the encoding of the XML file found in the first lines of the file (use Linux or Git for Windows you can do "head myXMLfile.xml"
2. You can definitely hack your way down the long long road of using Line-based importer, but I would not suggest it.
4. Try to find a tool that can tell you where the XML file is broken or invalid and manually clean it up (awk, sed, or my prefered tool, Altova XMLSpy which has a free trial)
I could go on, but I'll stop there and await your response.