I just wanted to get back to this thread with what issue I was facing, and how I solved it (in case a future Googler lands here):
While the initial Trust Failure was successfully resolved with the method @drwtsn32 suggested, I had an additional 401 - Unauthorized error appearing. The error message was quite confusing, as 401 normally indicates an HTTP(S) error, but in fact the unauthorized error I got was from when trying to access files over SMB. At some point I must have changed some permission setting, making Duplicati unable to access the mounted /source directory. It was just unfortunate that those two errors came after one another.