(First, I uninstalled the old GUI. It uninstalled, but it did not remove its Duplicacy Cloud Backup service. I had to remove that service manually.)
Next, I installed the Windows x64 Web UI. Ran it as administrator, installed for all users. Seemed to complete fine, but when I started to add storages, it complained that there was no CLI. Okay, I forgot to open my firewall, and it blocked something right when the Web UI started the first time. I opened the firewall (allow everything) and tried again, no good. I rebooted, no good. I manually got the latest CLI (2.7.2) from github and put it in the .duplicacy/bin folder. I needed to reboot again, but now all is well.
I don’t know what causes Web UI to try again to get the CLI executable, but rebooting and restarting Web UI didn’t do it. I’d like to suggest a Retry button to force this process. Even better, if the Retry fails, print the message that gets written into duplicacy_web.log on screen in an error dialog.