Yes, 8200 is the DEFAULT port - but when dealing with multiple “users” (including the service, if running) startup order can cause things to get messy.
Duplicati tries different ports, starting with 8200. The first user to log in will get that port, the next will get port 8300, next on 8400. The port number is recorded in the settings database, such that the next launch will attempt to keep this number.
This can go wrong, say that Kees is 8200, Ken is 8300, but then the machine reboots and Jon logs in for the first time. He will then get 8200, and when Kees logs in, Duplicati will see that 8200 is taken and scan for a new one, finding 8300, e…