And that's it. I installed phpmyadmin with the help of ryecoaaron's guide in this thread about phpmyadmin. It works wery well. I definitely suggest using phpmyadmin to the people who has a problem with MyWebSQL like me. And by the way it doesn't have to be the mysql db but also mariadb. It's more performant somewhere.
Thank you again guys (although we couldn't still find the reason why MyWebSQL doesn't work. 
but that's ok). 
I hope so.
Just don't install the phpmyadmin package from the Debian repos. You will have to have the root password problem worked out to get phpmyadmin to work since it needs to create a database to install. Can you login with mysql -u root -p?
Yes I can login to mysql with root password over ssh. I can execute the queries, see the tables and rows. Don't have problem with the actual database and login.
The problem is to login over web user interface MyWebSQL. Because I don't want to work always over terminal.
This kind of interfaces like phpmyadmin or mywebsql are more effective. I don't use db only for the services running on omv but also for some other purposes like my homepage or my web applications.
Just don't install the phpmyadmin package from the Debian repos. You will have to have the root password problem worked out to get phpmyadmin to work since it needs to create a database to install. Can you login with mysql -u root -p?
Yes I can login to mysql with root password over ssh. I can execute the queries, see the tables and rows. Don't have problem with the actual database and login.
The problem is to login over web user interface MyWebSQL. Because I don't want to work always over terminal.
This kind of interfaces like phpmyadmin or mywebsql are more effective. I don't use db only for the services running on omv but also for some other purposes like my homepage or my web applications.
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I really enjoy spending hours with omv. Totally amazing world for me.
But some problems cause not spending but wasting the time. That's what I mean.
You can use your time effectively by finding and using an alternative solution for some kind of problems.
I will post the results this evening.
Thank you again kawu2 and ryecoaaron.
I agree.
I agree, too. But the problem is to find that out.
Anyway, I will try the phpmyadmin this evening. Then I post the results here. I don't want to spend more time to resolve this problem of MyWebSQL.
Thank you for all your helps und time.
That is actually a good alternative option. I can try it this evening. But I don't really understand why this MyWebSQL doesn't work.
Hmm. Are the users saved in mysql library /var/lib/mysql in some kind of conf file? Or are they in the "user" table in the mysql database.
Or you mean the /var/lib/mysql is the storage location of the db tables?
As I wrote at the first message, I tried it as well but it didn't work.
When I try to login root with a wrong password, it appears an error message like "login or password is false" or something like that. The login and password input fields don't disappear and the system requires the password again. But when I try login with the right password, the page (sorry but I just realized that the message was in german) on the attachment of my first message on the top comes. When I reload the page to get the login fields again, the system shows me the same error message. Because it saves in the session, that I typed the right user and password. Only if I delete the firefox browser cache, I can reach the login page.
I mean, in my opinion the system finds out that my password is right. However it doesn't connect to the database. And that's why I get the error message. (Datenbank-Verbindung zum Server fehlgeschlagen. -Database connection to the server failed.)
The MyWebSQL can not have a separate table in DB, where it checks the passwords in addition to allow showing the WebGUI. Am I right ???
No unfortunately it didn't... 
I still get the same error.
Hi kawu2,
thank you for your answer.
I did exactly the same prozess like you said. I removed the whole packages with purge and installed them again. But it didn't solve my problem.
- I'm 100% sure you need to update passwords in MySQL database -
I updated root passwords via WebGUI as well as via Terminal mysql.
Of course you are right, I wouldn't change the root password as well, if I would have any services running on it. But I had no services running with mysql at that time i changed the root passwords.
Hi everyone,
yesterday I installed MySQL plugin and enabled the Web Interface. Then logged into MyWebSQL with User: root and without password successfully. At that time I didn't change any password or any setting. Today I want to log in again. But I couldn't.
Although I changed the root password of the four users (root@127.0.0.1, root@::1, root@myserver, root@localhost) over terminal (with the help of the page here), I can not log in and get the error message "Datenbank-Verbindung zum Server fehlgeschlagen. Host: localhost, Benutzer: root" (screenshot attached).
Today I spent several hours for solving this problem but I couldn't. Could you help me please?
Thank you.
