Thank you for contacting us and I'm sorry about the issue you are experiencing. What version of mSecure are you trying to use here?
Took me awhile to find but version 1.0.42 is labeled that on Windows Phones. That is version 3.5.4 of our old Microsoft Store app. Dropbox updated their code last year around September and that version of mSecure can no longer sync via Dropbox. I'm sorry I don't have better news here.
On my Windows 10 phone I get the message:
Undefind error while auto-syncing is gasping. You can turn off auto-sync and switch it on again later in the Settings menu.
There will be no contact with Dropbox. What to do