bridge relay with dirport enabled - it does not work!

[Gitano <0xb8968b9e9190@xxxxxxxxxxxxxx>]

When I set up my Tor server as a normal relay all works fine:
###
Jun 07 18:16:18.673 [notice] Now checking whether ORPort *.*.*.*:443 and
DirPort *.*.*.*:9030 are reachable... (this may take up to 20 minutes --
look for log messages indicating success)
Jun 07 18:16:24.684 [notice] Self-testing indicates your DirPort is
reachable from the outside. Excellent.
Jun 07 18:16:28.033 [notice] Self-testing indicates your ORPort is
reachable from the outside. Excellent. Publishing server descriptor.
Jun 07 18:16:54.674 [notice] Performing bandwidth self-test...done.
###

But when I set it up as a bridge relay

  BridgeRelay 1
  PublishServerDescriptor bridge

the DirPort is no longer reachable:
###
Jun 07 18:34:24.486 [notice] Now checking whether ORPort *.*.*.*:443 and
DirPort *.*.*.*:9030 are reachable... (this may take up to 20 minutes --
look for log messages indicating success)
Jun 07 18:34:28.792 [notice] Self-testing indicates your ORPort is
reachable from the outside. Excellent. Publishing server descriptor.
Jun 07 18:35:27.691 [notice] Performing bandwidth self-test...done.
Jun 07 18:54:19.312 [warn] Your server (*.*.*.*:9030) has not managed to
confirm that its DirPort is reachable. Please check your firewalls,
ports, address, /etc/hosts file, etc.
###

In the (debug-)logfile I found this:
###
Jun  7 18:34:25 server Tor[19309]: directory_handle_command_get():
rewritten url as '/tor/server/authority.z'.
Jun  7 18:34:26 server Tor[19309]: connection_dir_client_reached_eof():
Received response from directory server '*.*.*.*:9030': 404 "Not found"
Jun  7 18:34:26 server Tor[19309]: connection_dir_client_reached_eof():
Received server info (size 0) from server '*.*.*.*:9030'
Jun  7 18:34:26 server Tor[19309]: connection_dir_client_reached_eof():
Received http status code 404 ("Not found") from server '*.*.*.*:9030'
while fetching "/tor/server/authority.z". I'll try again soon.
###

Nothing else has been changed except the two lines in '/etc/tor/torrc'!

What am I missing? Thanks in advance.