mariadb won't start after upgrade

we have upgrade a SLES 12.1 to 12.5. After the update the installed mariadb won’t start.
The database wasn’t upgraded. So we started mariadb with “mysqld_safe”, did the upgarde of the tables,
but now we get the error message:

mysql-systemd-helper[28790]: chmod: changing permissions of '/usr/local/mysql/data': Read-only file system

The filesystem IS readable!
Also we can start the database using

/usr/lib/mysql/mysql-systemd-helper start

The everything is working and the DB can be accessed. However we do not understand why
“rcmysqld” restart or “service mysql restart” is not working.

How can we debug this problem ?
Any idea is well come!

Bye, Peer

OK - it seems the non standard location of the datadir caused the problem.

# Prevent writes to /usr, /boot, and /etc

# Prevent accessing /home, /root and /run/user

fixed the problem. Now everything works well.