Never mind, I solved it. Major PEBKAC.
I set two-factor auth on my QNap NAS, which turns out to be a bad idea. Among other things, Duplicati could no longer find it as an SFTP server. So I backed out that setting.
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I’m not familiar with QNAP two factor authentication - what is involved and do you think it’s something that COULD be added to Duplicati?
Oh, and I updated your topic title to indicate it’s a QNap NAS. Would you mind flagging your post as the solution - just in case somebody else runs into the same PEBKAC. 
(I can’t seem to do it for some reason…)
Neither can I. You can just delete it.
Huh, looks like maybe Discourse doesn’t allow the first post to also be the solution (maybe due to self referencing issues).
It’s not a big deal, I’d rather have the info recorded without the flash than delete it. 