Hello jurgenhaas and Bernie_O,
i tried the same on my .htaccess and now the error-messages in admin-panel are gone, redirection works; thank you!
But i still can’t follow other fediverse-instances and get a missing resource on blank screen (as above in 3 and 8) for app/social.
I will follow the issues on https://github.com/nextcloud/social/issues/ as it now seems to be an issue of app/social.
Thank you all. (Do i have to close this issue, an if so: how?)