Some news: I installed a new “My Webapp”, from scratch. I uploaded my phpBB forum in here and tried to do a fresh install. However, when giving the new username, password and database name, I have the same error:
Could not connect to the database, see error message below.
Access denied for user ‘my_webapp’@‘localhost’ (using password: YES)
[phpBB Debug] “mysqli_real_connect(): (HY000/1045): Access denied for user ‘my_webapp’@‘localhost’ (using password: YES)” in file /var/www/my_webapp/www/phpbb/db/driver/mysqli.php on line 64
I see I’m not alone in this: https://forum.yunohost.org/t/beginner-question-about-mysql-database-error-connecting-in-my-webapp 