I have also had trouble opening encrypted Duplicati-server.sqlite files in other tools. In my case I attempted to use DB Browser (SQLCipher) but it was not able to read it.
Just curious - would you be ok if the database was unencrypted on your machine? You can run Duplicati on Windows with an unencrypted database. If you are confident with the security of the actual file on your PC, and it’s more important that you be able to analyze the database with third party tools, maybe you’d prefer to go this route.
If you want to go that route, then according to this post you can have Duplicati decrypt your database for you.