Welcome to the forum @pcgear
This is possibly a TLS problem between mono and box.com.
What does mono --version say? If it says 4.6.2.7+dfsg-1 you might be on the latest from Debian but not high enough to do TLS 1.2. Before mono 4.8, TLS ended at TLS 1.0, leading to the following problem:
Ensure your apps are TLS 1.2 compliant by May 13th, 2019 - All TLS 1.0 traffic will be blocked
You can update mono here which is also mentioned as Duplicati Prerequisites and will be important soon.
There was an unfortunate bug in mono 6 that breaks OneDrive, but I think going to box.com would be OK.
If your mono is something recent, e.g.a 5.x or 6.x, then maybe changing –allowed-ssl-versions is needed.
Hopefully some of these changes will fix it. Please let us know which of the changes you wind up applying.