The ctor ‘new Document(String filename)’ opens a template file read-only and normally shouldn’t cause such an exception (if you don’t modifying the template doc in some another thread). So, please check did you modifying the template doc in any thread (including open Word editor).
Also please provide me your full stack trace - com.aspose.words.FileCorruptedException in this ctor wraps a range of different exceptions from IOException to NullPointerException – but we should to know the real cause.
And at last, for workaround you can try another Document ctor: Document(InputStream stream) instead of Document(String filename). The last one uses RandomAccessFile internally – may be there are some conflicts occur in your specific environment when you are using this class in multithread.