Ok, i solved my problem.
The IP Address (syslisteners) of my server is different from the host ip address.
I added the ASE server's IP Address to the hosts.allow file and then the dataserver can connect to the backupserver.
The backupserver error message is misleading. It shows the ip address of the host machine and not the ip address of the server while it's the last one that is necessary in the hosts.allow file.
Luc.