Hey @christin,
So for the correct start of the game, it is necessary to check the files and get the updates. Please, press the "check" button in the launcher before starting the game but if the previous solution didn't help, please, re-download our launcher: https://euroaion.com/files/euroaion.exe, with all of these changes you should be able to join our server, let me know if your problem got solved.
Regards Christof.