Hey Agriesser,
thank you so much for your answer.
So, I decide, that I´ve installed it NEW and deleted the old Version completely. And it’s work fine now.
BUT!
One question is coming out:
I have installed a NGINX proxy server in front of the Mattermost server and the redirection works perfectly.
However, what do I have to enter under the system console of Mattermost? At least Mattermost asks for this every time I log in as “Administrator”.
Kind regards
Z. Matthias