directly after a fresh Neos installation (and choosing the Neos Demo Site Package) the frontend crashes with this error:
Call to undefined function GuzzleHttp\Psr7\str()
The Backend login works (despite of that the frontend preview shows the same error).
Tested locally on a PHP7.4 and on a Webserver with PHP7.3 and the same Error.
The installation method was with:
$ composer create-project neos/neos-base-distribution project