Can't login to the backend of a fresh TYPO3 install

Here is what I did:

1. Downloaded zip TYPO3 11.3.0 from https://get.typo3.org/version/11
2. Unzip into /subfolder
3. touch FIRST_INSTALL
4. Followed instructions of the setup wizard with new database
5. Try to login via /subfolder/typo3/ and enter credentials
6. /subfolder/typo3/login?loginProvider=... leads to a 404

What I already checked:

• typo3temp/var/log/typo3_[...].log is empty
• MAMP logs don't show anything suspicious
• Install Tool Environment Status shows a weird error: http://localhost:8888/subfolder/typo3temp/assets/e35bd633.tmp/e77fab4e.php.wrong: unexpected content PHP content
• Install Tool Directory Status is all green
• Original .htaccess file exists
• Created typo3conf/sites/mysite/config.yaml by copying from an existing (working) project, TYPO3 10

Of course, before I created the site configuration YAML, /subfolder/typo3/login?loginProvider=... was leading to a "no site config" error.
Now, it feels like there is a problem with the .htaccess, but I can't pinpoint where.

Running Apache / PHP 7.4.12