Thank you for contacting us, and I'm sorry for the issue you are experiencing. Unfortunately, Dropbox syncing is no longer working in the Windows version of mSecure 5. It's fixed in mSecure 6, which is close to release and in beta testing now, but it won't work in mSecure 5. We are recommending our users switch to mSecure Cloud syncing instead. That will always work moving forward, and if for whatever reason you wish to use Dropbox syncing instead, you can switch over to that when mSecure 6 becomes available.
Since you have a Pro license, you will continue to have access to the Pro features in mSecure 6 when it's released. There will be other new features that will require a purchase should you decide to use them for longer than the 30 day trial period, but you won't lose access to the features you purchased for mSecure 5.