Hey all,
for me, I was trying to access the SLM from a remote computer, not the Core Server.
In testing, I copied the link the Core Server gave me and typed it into the browser of another machine which gave me this error as well.
Instead, I consolidated the link down from what appears in the core server to: https://CoreServerName/console/SLConsole.aspx
For me, removing everything after the .aspx in the URL allowed me in to the webpage and I was able to use SLM from a remote computer.
Hope this helps with your issue as it's a simple fix!