Ok ,that makes sense.

I now created a new task and removed the slave, the task is scheduled but keeps on 'Requested' state.
The logfile shows:
ad admin:UNKNOWN:2011-05-10 20h39.48 CEST:6362: file /builddir/build/BUILD/openvas-administrator-1.1.1/src/admin.c: line 921 (openvas_get_sync_script_description): assertion failed: (*description)
==> /var/log/openvas/openvasmd.log <==
event auth:MESSAGE:2011-05-10 19h48.35 utc :10598: Authentication success for user admin (b2316fa2-1cac-4023-a26e-f8d10407e5e6)
event auth:MESSAGE:2011-05-10 19h48.50 utc :10601: Authentication success for user admin (b2316fa2-1cac-4023-a26e-f8d10407e5e6)
event task:MESSAGE:2011-05-10 19h48.50 utc :10601: Status of task (ace59c1c-29b4-4f9c-a17a-1cf0d32bc556) has changed to New
event task:MESSAGE:2011-05-10 19h48.57 utc :10601: Task ace59c1c-29b4-4f9c-a17a-1cf0d32bc556 has been created
event auth:MESSAGE:2011-05-10 19h50.01 utc :10676: Authentication success for user admin (b2316fa2-1cac-4023-a26e-f8d10407e5e6)
event auth:MESSAGE:2011-05-10 19h50.13 utc :10681: Authentication success for user admin (b2316fa2-1cac-4023-a26e-f8d10407e5e6)
event task:MESSAGE:2011-05-10 19h50.13 utc :10681: Status of task test2 (ace59c1c-29b4-4f9c-a17a-1cf0d32bc556) has changed to Requested
event task:MESSAGE:2011-05-10 19h50.23 utc :10681: Task ace59c1c-29b4-4f9c-a17a-1cf0d32bc556 has been requested to start
lib serv:WARNING:2011-05-10 19h50.23 utc :10681: Failed to gnutls_bye: Error in the push function.
==> /var/log/openvas/openvassd.log <==
[Tue May 10 16:30:50 2011][2129] received the TERM signal
[Tue May 10 16:48:41 2011][4551] openvassd 3.2.3 started
[Tue May 10 20:47:04 2011][6414] user om starts a new scan. Target(s) :
www.info.nl, with max_hosts = 20 and max_checks = 4
[Tue May 10 20:47:04 2011][6414] user om : testing 80.79.193.95 (::ffff:80.79.193.95) [6554]
[Tue May 10 20:47:04 2011][6554] user om : new KB will be saved as /var/lib/openvas/users/om/kbs/80.79.193.95
[Tue May 10 20:53:10 2011][6554] Finished testing 80.79.193.95. Time : 365.75 secs
[Tue May 10 20:53:10 2011][6414] user om : test complete
[Tue May 10 20:53:10 2011][6414] Total time to scan all hosts : 367 seconds
[Tue May 10 20:53:10 2011][6414] user om : Kept alive connection
[Tue May 10 20:53:10 2011][6414] Communication closed by client
==> /var/log/openvas/openvasmd.log <==
event auth:MESSAGE:2011-05-10 19h52.36 utc :10704: Authentication success for user admin (b2316fa2-1cac-4023-a26e-f8d10407e5e6)
event auth:MESSAGE:2011-05-10 19h52.47 utc :10707: Authentication success for user admin (b2316fa2-1cac-4023-a26e-f8d10407e5e6)
event auth:MESSAGE:2011-05-10 19h52.59 utc :10710: Authentication success for user admin (b2316fa2-1cac-4023-a26e-f8d10407e5e6)
event auth:MESSAGE:2011-05-10 19h53.15 utc :10724: Authentication success for user admin (b2316fa2-1cac-4023-a26e-f8d10407e5e6)