Sure, but none of the versions work. They all result in the same thing. This seems not to be the problem. May I ask what is the standard version for Buster 10?
Look in the mysql error log and other log files (eg. daemon or syslog), it should say the reason why. Sometimes it's a bad config setting, sometimes corrupt db tables, etc. If you can't find any error in a log, you could try running the mysqld commandline from a shell and it might print an error there.
Now I have reinstalled the VPS and it is like it was before. It can even handle the forced update. I am not sure but maybe (?) it's related to that I set a password during the MYSQL_SECURE_INSTALLATION and then I noticed that the "-p" switch wasn't there (check one of the screenshots). Now everything is up and works like a charm, EXCEPT that the Resync tools are with even fewer options. I will try something else now. Thank you everyone who has tried to help me so far. I really appreciate it!
You can also use the autoinstaller to make sure you don't do anything wrong: https://www.howtoforge.com/ispconfig-autoinstall-debian-ubuntu/
The Resync problem, I know what is wrong now. It's actually not a two-way sync as I understand it, rather a one-way. It says this in the manual: ...but this seem not to be correct. In order to be "synched" they have to be created in ISPconfig. I mean, the database and users belonging to it, must be registered in ISPconfig3 for ISPconfig3 to offer Resync for it. The above, yellow marked, from the manual seem not to be correct. I mean, this is exactly what I did so my new installation of ISPconfig3 was not aware of that there were databases and database users. I did the same with the websites, same there. They didn't show up as syncable services because ISPconfig3 didn't know of them. Now I tried to add one website manually and -tada!- Resync of websites was suddenly available. Since a database is associated with a website, all websites must be created before I can even get the resync to show up as a synchable service. Whether it will synch or not is unknown for now. I will get back on the matter. If I may ask for something, it would be to make ISPconfig work as it says in the manual or, if this is me not having English as my mother tongue, then perhaps I just need to read more carefully, but to me it does seem to be like I interpreted it.
Is there a way to extract the created (in ISPconfig3) websites, databases, mailboxes etc from the source server, the settings I mean? Again, thank you all for your help in this! It's much appreciated!
The manual tells you what it does: recreate all config files based on the database. For migration, take a look at https://www.ispconfig.org/add-ons/ispconfig-migration-tool/
Yep, I know about the migration tool, I have bought it before. It messes up the structure. A client with ID 1 might get a migrated database with ID 4. It's confusing when having many sites. Unfortunately.
Sorry to say, but I don't see that. Unless it is already created in ISPconfig3 it just doesn't read what is on disk and get that into ISPconfig3. That is the problem here.
I just tested it. I placed a database I got from the source server in /var/lib/mysql/ . It was unknown to ISPconfig3 in the new server. Since I had created another database using ISPconfig3, Database synch showed up as a synchable service in ISPC3. When I ran the Resync it only resynched the database that was registered via ISPC3, not the database I placed directly in /var/lib/mysql (which was unknown to ISPC3).
If I had not created a database at all in ISPC3, then Resync database would not be in the list of Resync Tools.
I was talking about the migration tool, which works fine. Of course nothing was done with the database that was not added in ISPConfig. When you use ISPConfig, config files are generated based on the settings for the item. With a website for example, a vhost is created based on the settings in for the website. If you add a random vhost file, ISPConfig usually doesn't touch that at all - is it not known to the software because it is not in the database. See my explanation above.
One more note, before you edited your comment, it said "Sorry to say, but I don't see that. Unless it is already created in ISPconfig3 it just doesn't read what is on disk and get that into ISPconfig3. That is the problem here." As I explained above, this would not work and never has worked, nor have we claimed anywhere that this would be the case.
Ok, but that is how I read it in the manual. I guess I have to blame that I am not native English speaking.