"Internal Server Error" after update from 21.0.2 to 21.0.4

I updated my docker-compose NC instance from version 21.0.2 to 21.0.4 using the GUI-updater. Now I see " Internal Server Error" when browsing to the login-page.

within the container, it tells me that the update succeded:

Nextcloud Updater - version: v20.0.0beta4-11-g68fa0d4
Current version is 21.0.4.
No update available.
Nothing to do. 

The NC-logfile: https://pastebin.com/VXJF8HZw

This might be the critical line from the nginx-error.log:

2021/08/23 13:24:20 [error] 422#422: *124 connect() failed (111: Connection refused) while connecting to upstream, client:, server: nextcloud.mydomain.com, request: “GET /apps/dashboard/index.php/csrftoken HTTP/2.0”, upstream: “”, host: “nextcloud.mydomain.com”, referrer: “https://nextcloud.mydomain.com/apps/dashboard/

Could you please help me to troubleshoot this? I’d like to get NC up and running again aspa… Thank you!

It turned out, that there was a unknown reference in a class of Guzzle. occ app:list within the NC-container revealed the error. Ich changed from the variable in
/config/www/nextcloud/3rdparty/guzzlehttp/guzzle/src/Utils.php line 108 from
return sprintf('GuzzleHttp/%d', ClientInterface::MAJOR_VERSION);
return sprintf('GuzzleHttp/%d', ClientInterface::VERSION);
and this did the trick.