Thanks! I’ve successfully downgraded!
What I did :
- download http://sqlitebrowser.org/ (following @Pectojin advice)
- launch this software and open “Duplicati-server.sqlite” file. It’s a SQLlite database, so it doesn’t support “DROP” command, but you can follow these indication to remove “LogEntryID”, “MessageID” and “MessageLogTag” fields : https://stackoverflow.com/a/48097199/6620863
- with the same software, modify the version value that you will find in “Version” table (“4” instead of “5”)
- save the database modifications
- install and launch Duplicati Beta