Hello Lupopa und willkommen bei jOpenSim
Allthough I'm austrian and also able to reply in "ösi-german", I prefer to stick to english as other users might find this info useful as well
Usually you have a mysql user for OpenSim like e.g. opensim@localhost that needs to be entered into the config-include/GridCommon.ini in the section [DatabaseService]. If the webserver, where Joomla and jOpenSim is located is on a different server (e.g. out-world.eu), MySQL will not allow access for it like this. So best practice is to duplicate opensim@localhost to
to give access for jOpenSim.
Mind also, that in most cases the port of MySQL (usually 3306) is not opened for outside (which is actually - for security reasons - no bad idea). How to change firewall for this depends very much on your system. For my part, I prefer to open this port only for the IP address of the webserver.
Grüße
FoTo50