Backup to FTP with TLS/SSL produces error with self-signed certificate

I am trying to get Duplicati to back up to a FTP that uses TLS/SSL (VSFTPD on Linux) and am getting a error message every time the job runs.

I am using a self-signed certificate on the FTP server, but even if I tell the Duplicati to accept any SSL certificate I still get "Error while running , The remote certificate is invalid according to the validation procedure."
I’ve tested with accept-specified-ssl-hash too, makes no difference.
The Test Connection button reports all is well and working.
The backup does run though, and I can restore files from it (but it produces a similar error).
I think Duplicati is not truly trusting the certificate, even when told to.

The job log is not showing anything useful, but the Duplicati log itself shows:

System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure. at Duplicati.Library.Main.Operation.BackupHandler.HandleFilesystemEntry(ISnapshotService snapshot, BackendManager backend, String path, FileAttributes attributes) at Duplicati.Library.Main.Operation.BackupHandler.RunMainOperation(ISnapshotService snapshot, BackendManager backend) at Duplicati.Library.Main.Operation.BackupHandler.Run(String[] sources, IFilter filter) at Duplicati.Library.Main.Controller.<>c__DisplayClass16_0.<Backup>b__0(BackupResults result) at Duplicati.Library.Main.Controller.RunAction[T](T result, String[]& paths, IFilter& filter, Action`1 method) at Duplicati.Library.Main.Controller.Backup(String[] inputsources, IFilter filter) at Duplicati.Server.Runner.Run(IRunnerData data, Boolean fromQueue)

VSFTPD is set ot only accept TLS/SSL connections (force_local_data_ssl=yes, force_local_logins_ssl=yes)

If I switch to SSH it works fine…

Seems this post talks about the same issue:

And it appears to be a bug, so hopefully it will get fixed soon.