Craft CMS throws Internal Server Error 500 I got given a Craft CMS project build with composer and npm. Pointed valet to the correct folder, installed all the dependencies and started the app with npm run watch.
http://localhost:3000 just gives me an &quot;Internal Server Error&quot; though...

There is nothing in the logs apart from sth inside storage/logs/web.log which does not sound like a showstopper:
2020-12-07 06:59:06 [-][-][-][error][yii\base\ErrorException:8192] yii\base\ErrorException: Method ReflectionParameter::isArray() is deprecated in /vendor/yiisoft/yii2/web/Controller.php:133 Stack trace:
Error 500 bugs are really hard to track down, any hints would be appreciated!
Based on the error message and this: https://php.watch/versions/8.0/deprecated-reflectionparameter-methods, it sounds like you're running PHP 8, which Craft isn't compatible with, yet.
The soon-to-be-released Craft 3.6 adds PHP 8 compatibility, so you'll want to downgrade to PHP 7.4 until that's out.