amcheck fails

Hello everybody,
I have a backup solution up and running for all the enterprise servers based on a 2.4.5 server et 2.4.5 client under Gentoo.
The latest server delivered are under RHEL 5. So I installed the oldest client-backup I found: 2.5.1.
And when I install the rpm, it created everything for user amandabackup. As all the other servers are using user amanda, I am trying to modify the configuration to use this user (amanda).
When I launch amcheck from the server, I receive following error: ERROR: NAK almerysw: getpwnam(amandabackup) fails

Looking at amandad logs of the client, I find this:
amandad: time 0.000: CONFIGURE_COMMAND="'./configure' '--build=i386-redhat-linux' '--prefix=/usr' '--bindir=/usr/bin' '--sbindir=/usr/sbin' '--libexecdir=/usr/lib/amanda' '--datadir=/usr/share' '--sysconfdir=/etc' '--sharedstatedir=/var/lib/amanda' '--localstatedir=/var/lib/amanda' '--libdir=/usr/lib' '--includedir=/usr/include' '--infodir=/usr/info' '--mandir=/usr/share/man' '--with-gnutar=/bin/tar' '--with-gnutar-listdir=/var/lib/amanda/gnutar-lists' '--with-dumperdir=/usr/lib/amanda' '--with-index-server=localhost' '--with-tape-server=localhost' '--with-user=amandabackup' '--with-group=disk' '--with-owner=paddy' '--with-fqdn' '--with-bsd-security' '--with-bsdtcp-security' '--with-bsdudp-security' '--with-ssh-security' '--with-assertions'"

So I think that the problem is with the option --with-user=amandabackup

Where can I modify the behavior of amanda so that it runs with amanda user?
Is it possible to install the rpm configured to use amanda username?

Thanks in advance for your help. I already missed 2 backups, it's getting urgent...
I hope I gave enough details, don't hesitate to ask if something is missing.

Have a nice day

Pierre