NC connection wizard tries to access token endpoint via http instead of https

I have a working Nextcloud 18.0.3 installation on my synology server (OS: DSM 6.2.2-24922 Update 4) which is already connected to 3 clients including my smartphone. Now I would like to connect an additional Windows 10 Home 32 bit client
The browser access (like works fine from that machine.
When connecting with the connection wizard of nextcloud client (2.6.4 stable Win 32)
I see this error message.

    Fehler beim Zugriff auf den Token' -Endpunkt:
    Error transferring - server replied: Bad Request

The wizard tries to use http instead of https. However my nextcloud server is not reachable via http from internet.
How to overcome the blockage?

REM 1: As a possible outcome I already had tried to find out how to rewrite http access to https on Nextcloud side, but my Google search was not successful here. Actually, for me that is not so important as the Nextcloud instance is for private use mainly. And as far as I remember there was a form in the earlier connection wizard where I could enter the correct URL with leading https://
REM 2: As I can access Nextcloud in my local network with http://192.168…:8090 I used this as a workaround. But obviously this is a local network solution only. Probably I could configure my router to forward the external requests to the http port, but that is not what I want due to security concerns.
Over all the token access via http could be a bug.

Ideally you have a redirect for https. Not sure why it doesn’t use https per default (and http as fall-back). The login-flow documentation uses https:

You’ll probably have to ask the developers:

Hi Tflidd
Thank you very much for your reply
I checked login-flow documentation chapter ‘Login flow v2’
That is all about https.
And in the German installation guide I used one year ago ( ) I found this:
“Mit einem ReverseProxy-Eintrag im NAS leitet nginx HTTPS Verbindungswünsche an den Nextcloud Container weiter, der selbst nur HTTP spricht”
(“With a reverse proxy entry in the NAS, nginx forwards HTTPS connection requests to the NextCloud container, which itself only speaks HTTP”)
Probably this reverse proxy is misconfigured not the wizard.

Just started getting this today - no problems with android clients just Linux desktop clients. Suddenly becomes unusable. No other changes to networks that I am aware of.