Thank you very much for answering my question!
The execution of the commands provided by you was successful, but unfortunately nothing has changed in the result.
A new execution of the update management in the Web control panel brings the following information:
W: The repository 'https://openmediavault.github.io/packages arrakis Release' does not have a Release file.
W: The repository 'https://dl.bintray.com/openmediavault-plugin-developers/arrakis stretch Release' does not have a Release file.
E: Failed to fetch https://openmediavault.github.…ain/binary-amd64/Packages gnutls_handshake() failed: Public key signature verification has failed.
E: Failed to fetch https://dl.bintray.com/openmed…ain/binary-amd64/Packages gnutls_handshake() failed: Public key signature verification has failed.
E: Some index files failed to download. They have been ignored, or old ones used instead. in /usr/share/openmediavault/engined/rpc/apt.inc:220
Is there any additional information I could provide for a possible solution?
Thanks in advance