Comment 5 for bug 1812912

I think this was due to a few bugs working in concert, causing situations where your backup chain was encrypted with two different passphrases. See the following wiki page to see if you are affected and how to recover.
https://wiki.gnome.org/Apps/DejaDup/PassphraseProblems2019

I will optimistically close this as “fixed in 39.1” but please reopen if you believe this to be caused by something else.