With OMV3 there was a problem with certificates when using Duplicati as plug-in in OMV3 with Amazon Drive.
This has now been solved with OMV4 which is based on Debian 8 (Stretch). OMV4 is not announced as stable, yet, but is already used by many users.
Background: when the Duplicati plug-in is installed on OMV4 ca-certificates-mono is installed. This was not the case with OMV3 as ca-certificates-mono was not included in standard Debian 7 repo.