The "Quicken an unknown error occurred" happens due to multiple reasons such as slow internet connection, outdated version of Quicken application, and slow window's response. To fix this error, you will need to update Quicken and to do that first go to help section and click about us, and from there, you can download the latest Quicken application update. Now run the .exe file and install it on your computer by clicking Run. Now you can start Quicken application and work with it trouble-free. If the problem doesn't resolve, then you can uninstall and then reinstall the app on your computer, this step will help you fix the bugs in your Quicken application as well.