We have changed our Sandbox at https://sandbox.mapbender.org. Formerly
you could register yourself and play around with it. Now we provide a
demo-account to access the administration backend of Mapbender.
Use demo/mapbender as login-credentials.
If you ever had a login there, they are deleted. Take care. As our main
website (https://mapbender.org/?q=en/demo) states, the sandbox is
cleaned up regularly anyway.
We check again if we need the old register functionality but I guess it
might also work with this demo account.