if you are using an Azure AD (MS365) joined machine, you might experience problems when trying to connect to a non-domain joined machine. If this is the case, there are two possible solutions to this problem. The first is to edit the RDP Plugin settings under "View - Plugins - Remote Desktop - Settings - Advanced". There, simply activate the "Set Domain to '.' for Local Accounts" setting:

Alternatively, you can also do this manually in the credentials for such a connection. Do do this, change the Username from "Username" to ".\Username" in the credentials and the domain will be omitted: