Last Mattermost on docker , and new problem

Hi there,
Installed mattermost (preview) on docker : no problem at all.
BUT I got "Please check connection, Mattermost unreachable. " ERROR (in red !) on the webpage.

I tried with some effort to use let’s encrypt and it works (partially) with nginx like a charm for other web app (NOn secure but Valid certificate.
Any ideas ? (tried to follow some thread here but no success)
Of course is on docker and in soho environment.
Thank you!

maybe solved I upload a solution but need to check.