Are there any clients currently connected to the AP through wireless? Otherwise aireplay-ng will naturally not be able to intercept any data packets and you will have to make sure that you use the correct method to crack WEP without any active clients.
There is no reason that aircrack-ng would be complaining about an issue with the packet length. The information about the error message is according to aircrack-ng.org: "It was due to the use of madwifi-ng with aircrack and aircrack-ng up to 0.2.1". The information is referring to the aircrack-ng suite as a whole, that is airodump and aireplay as well, and most likely the error did actually occur using aireplay-ng. As you neither use the madwifi drivers, or hopefully an version of aircrack-ng older than 0.2.1 it does however seem a bit puzzling that you are experiencing the error.



