Wagtail login downgrades to http, gives error

I am setting up a local, Dockerised version of a Django Wagtail project (which I have had running locally before). I am able to see Pages in my browser, but when I try to login (on FF, Chrome, Safari-with-no-plugins) at https://mysite.test/admin/login/?next=/admin/ I get the error:

gaierror at /admin/login/[Errno -2] Name or service not knownRequest Method:     POSTRequest URL:    http://mysite.test/admin/login/Django Version:     5.0.3Exception Type:     gaierrorException Value:    [Errno -2] Name or service not knownException Location:     /usr/lib/python3.10/socket.py, line 955, in getaddrinfoRaised during:  wagtail.admin.views.account.LoginViewPython Executable:  /root/uv/.venv/bin/pythonPython Version:     3.10.12Python Path:    ['/usr/srv/app','/usr/lib/python310.zip','/usr/lib/python3.10','/usr/lib/python3.10/lib-dynload','/root/uv/.venv/lib/python3.10/site-packages','/root/uv/.venv/lib/python3.10/site-packages/setuptools/_vendor']Server time:    Thu, 03 Jul 2025 13:44:22 +0000

which includes the request URL http://mysite.test/admin/login/.

I don't have a VPN running and I can't identify anything else which might be causing trouble on the local network (though I can't discount this).

If I try https://127.0.0.1:5000/admin I get:

Secure Connection FailedAn error occurred during a connection to 127.0.0.1:5000. SSL received a record that exceeded the maximum permissible length.Error code: SSL_ERROR_RX_RECORD_TOO_LONG    The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.    Please contact the web site owners to inform them of this problem.

Note: I have another Dockerised project (CiviCRM) running out of Orbstack on this system and it works fine, i.e. I can login on https.

What could be going wrong here?