"Invalid credentials"

[megalotee]

I read the old issue about this error, but it is closed and the suggestions did not help me resolve this:
I still get "Invalid credentials" trying to connect the local/master to the remote.
Installation on the remote works fine, but I cannot connect the local/master to it.
My connection requires https, so I always tick "secure".

So far, I tried:

• Configuration via the "add integration" dialogue ("Add a remote node"),
  which fails with "Invalid credentials" in the 2nd dialogue asking for host, port, etc.

• Manual editing of the 'master' configuration.yaml, e.g.:

  remote_homeassistant:
  instances:
  - host: <I tried remote FQDN & IP>
  port: <remote port forwarded to apache reverse proxy - The remote HA instance is usable by browser via this host:port >
  secure: true
  verify_ssl: false [ true results in "failed to connect to server" ]
  access_token: < remote-HA-generated token >
  entity_prefix: "rem_"

  ... this results in errors in the log shown below.

• removing the integration from the remote, rebooting, re-installing & trying again

• creating a new token, rebooting & trying again

Versions on BOTH HA instances:
Core 2024.1.6
Supervisor 2023.12.1
Operating System 11.4
Frontend 20240104.0

Error Log:
2024-02-01 20:51:41.865 ERROR (MainThread) [custom_components.remote_homeassistant.config_flow] import of failed
Traceback (most recent call last):
File "/config/custom_components/remote_homeassistant/config_flow.py", line 183, in async_step_import
info = await validate_input(self.hass, user_input)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/config/custom_components/remote_homeassistant/config_flow.py", line 45, in validate_input
info = await async_get_discovery_info(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/config/custom_components/remote_homeassistant/rest_api.py", line 51, in async_get_discovery_info
raise InvalidAuth()
custom_components.remote_homeassistant.rest_api.InvalidAuth

Any ideas?
Did I miss something silly?