This error occurs with the copy citation and generate bibliography features if you're connecting to us via a proxy service (you can check this by looking at the URL - if it contains a prefix like proxyname.workspace.leanlibrary.com you're connecting via proxy). You can work around it by opening a new tab and going to Lean Library Workspace directly (so the URL is just https://workspace.leanlibrary.com without having a proxy prefix in the URL) then using the desired feature there.