How the problem was finally solved (still thanks to anyone involved!)
Plesk's firewall was configured correctly, but IONOS-panel's own firewall wasn't.
The service xinetd (superserver responsible for things like FTP-server) wasn't started.
Also vsftpd was listening to port 21 as well (since IONOS guy told me to install it earlier in troubleshooting)
Now FTP is working and I can even create accounts in Plesk (not only SSH). Case closed
- Signed up for Plesk-Support (costs $9,99 a month, but first month is free, so $0)
- The support found out that it could connect until IONOS, so there had to be a problem there.
- In my provider panel's firewall settings, I've found that the checked configuration was not the one where port 21 (or ports 49152-65535/tcp) were open.
- I added these ports to the checked configuration
- The support also found out that "xinetd" was down and restarted it
- Beforehand he had to shut down "vsftpd", an extension I had previously installed since the IONOS-guy told me "there is no service behind port 21, you need to install one"
Plesk's firewall was configured correctly, but IONOS-panel's own firewall wasn't.
The service xinetd (superserver responsible for things like FTP-server) wasn't started.
Also vsftpd was listening to port 21 as well (since IONOS guy told me to install it earlier in troubleshooting)
Code:
# sudo systemctl stop vsftpd
# sudo systemctl restart xinetd
Now FTP is working and I can even create accounts in Plesk (not only SSH). Case closed