Thanks for your reply.
The different PHP versions were acutually causing the error. Composer was runnning with PHP 7.4.5 and the Neos Image with PHP 7.2. After fixing this problem (changing the PHP version in the Dockerfile), the setup was going further but crashed again.
I was able to set up the DB, the admin account and filling out the form to create a new site (Step 4 of 5). When I clicked on ‘Next’ there happend something but after a while the following errors were displayed on the screen:
**Fatal error** : Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in **/app/Packages/Libraries/composer/composer/src/Composer/Json/JsonFile.php** on line **288**
**Fatal error** : Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in **/app/Packages/Libraries/composer/composer/src/Composer/Console/Application.php** on line **87**
This error I fixed with the advices from Namone (https://github.com/nextcloud/docker/issues/224), that fixes the problem above but leads to the following error:
Once again, I have no idea, how to fix this problem.
[Edit and problem solved]
The exception was thrown, because I was hurrying thru the setup for the 20eth timeand chosing the Demo Site Package and entered some characters in the fields for a new Site Package as well. Simply doing one or the other was what solved the problem.