Time machine server / Bonjour

At my workplace we have Mac OS X Server Leopard 10.5.3 that we use as a Time Machine server, with a share point with the option "Enable as Time Machine backup destination" checked. Until now my Time Machine worked very well, both when my computer was connected to our lab's Airport or wired-ethernet.
But now there is a new institute-wide wlan, from which I can contact our server (with ssh, afp, ...). When I connect to this network Time Machine doesn't backup automatically anymore, and the backup fails. In my system.log I discovered the following:

/System/Library/CoreServices/backupd[3899]: Network mountpoint /Volumes/time not owned by backupd... remounting
/System/Library/CoreServices/backupd[3899]: [SnapshotUtilities remountVolumeRef] url could not be resolved via BonJour
/System/Library/CoreServices/backupd[3899]: Failed to remount network volume.
/System/Library/CoreServices/backupd[3899]: Backup failed with error: 19

So, apparently the problem is the fact that Bonjour is not working over the institute-wide network, which personally I find kind of logical, or at least it's difficult for me to find reasons to convince the upper levels to enable it. I also don't see any reason why Time Machine actually 'needs' bonjour, since it can connect the server, by ip address, hostname. And I can mount the time machine mount 'by hand' with 'Connect to server..." from Finder.
Is there anybody who knows (without destroying my current Time Machine archive) some way to change this behavior of Time machine so that it doesn't need Bonjour (by changing some setting, or putting a hard ip address in a plist or something) ?

Thx,

Werner Van Geit