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

[Selim]

You should uninstall the extension "compatibility7". Your problem probably will be fixed.
If you can not reach backend module to remöve "compatibility7" extension, then apply the solution offered by Christian (copying the TagBuilder.php file from typo3_src-7.6.x/typo3/sysext/fluid/Classes/Core/ViewHelper to typo3_src-8.7.x/typo3/sysext/fluid/Classes/Core/ViewHelper direcory).

Clear caches and try to login backend module. After login uninstall compatibility7 via extension manager. You can then remove the TagBuilder.php.

Selim