[TYPO3-linux] Upgrade from latest v7 to latest v8 -> BE Error: TagBuilder' not found

[Run DMC]

Hi all,

I upgraded latest v7 to v8 after the following manual:
 * docs.typo3.org/typo3cms/InstallationGuide/QuickInstall/GetAndUnpack/Index.html

I can log into the install tool, that works fine.
I went through the install tool upgrade wizard, all ok.

When I call the backend I get the following error:

Class 'TYPO3\CMS\Fluid\Core\ViewHelper\TagBuilder' not found 
Error thrown in file
/var/www/vhosts/hannahs-schwestern.de/httpdocs/typo3_src-8.7.1/typo3/sysext/fluid/Classes/Core/ViewHelper/AbstractTagBasedViewHelper.php in line 56.

And I wonder why ... there is really no (!) reason for this error to show up! ;-)

Search engines found nothing related this issue, what fixed my problem.

Some hints would be highly appreciated.