time machine only backups trough ethernet

Hello!

since i restored my mac after a hardware failure (sata cable in my Macbook Pro Mid 2009 died) i restored my mac using time machine to a new disk (first thought my harddrive quit working on me)

all went fine, but currently im only able to to my time machine backups trough ethernet, and not trough Airport.. when i initiate a manual backup i get;

Cannot complete the backup on 'TimeMachine' -> the time machine disk is not avaiable

when i open up the console i see;

21-11-12 09:01:06,685 com.apple.backupd[240]: Starting manual backup

21-11-12 09:01:06,908 com.apple.backupd[240]: Backup failed with error: 19

while the device is reachable trough finder and i can ping it! - and when i plug an a ethernet cable to the same network, backup works just fine.

any ideas?