I think the variable parts of the web page use a server download. That version is not hardcoded in HTML.
Why the download isn’t happening is a good question. Do you have anything interfering with JavaScript?
What else isn’t working, e.g. can you get anywhere with adding a backup (don’t need to actually save it)?
I’ve seen two Duplicati get same port before. Maybe run sudo netstat -ap | grep ':8200.*LISTEN'
EDIT:
You can compare HTTP network activity between working and the missing-version version using the web developer tools in Firefox. It’s probably F12 to open, go to Network, try loading the home page, look for the systeminfo file which looks like one of the first after the more fixed files like the many .js and some .png.
There’s a line to type systeminfo to filter for that. You can look at Response tab on right if it responded…