Alright, thanks for the help! I did that, and this time I used this answer to track down the log messages, which all look like this:
Oct 6, 2019 10:35 AM: The operation Repair has failed with error: Failed to decrypt data (invalid passphrase?): Invalid password or corrupted data
Oct 6, 2019 10:35 AM: Failed to process file: duplicati-20190612T205828Z.dlist.zip.aes
… etc. I am fairly certain I have the correct credentials, because I use a Password Manager and copy-paste all my passwords so it would be fairly difficult for me to mess it up. So, my best guess is the files got corrupted somehow.
Does that seem like a reasonable analysis? And if so, am I just out of luck? Thanks for the help!