Not that I see. But probably a good idea to lookup additional infos in this forum regarding “moving” data store to a new server (similar to server reinstall with existing repository) and uninstall client. I thought I found one. There is even somewhere an uninstall script. Ah found it: Howto fully uninstall urbackup client on Ubuntu
You could also look into the db if all clients are removed before you try to add them. I think there are also some hints in the forum about that.
Would be nice if @uroni had some hints for situations like this. Because I never found anything in debug logs on server and client. I am still not sure why they suddenly connected.