How to decrypt backup with asymmetric gpg encryption

Hi all,
I set up Duplicati to use asymmetric gpg encryption and the encryption part is working fine. I can also decrypt the *.gpg files from the backup location. The restore works fine if I unlock the private key before starting the process.
What I cannot manage is to restore the backup using the Duplicati web GUI without manually unlocking the key first. The process always fails because the private key required for decryption is locked. The default option passed to gpg by Duplicati is --passphrase-fd 0 which waits for input from STDIN but is obviously not available because the process is started from the web GUI.

I also tried to pass --passphrase "MySecret" with --gpg-decryption-switches but it did not work.

So how do I restore my backup from the web GUI without unlocking my key on the shell before starting the restore?