The installation script has apparently not initialised the database and has not created the files. This is why the daemon crashes (the return value 255 originates from the crashing script under bin/daemon.php).

My workaround:

Access the server via SSH

Switch to the Friendica environment with sudo yunohost app shell friendica

bin/console dbstructure update creates the database structure

Unfortunately, the user specified in the YunoHost installation is not available in the DB, which is why this user is also created on the console:

bin/console user add <username> <nickname> <mail-address> <language-code> where the mail address must be the one selected for the admin at YunoHost

Set password with bin/console user password <password>

You can then log in to the web as this user and have admin rights.

So there must be an error somewhere in the install script of the YunoHost app.