While it's not a fix I was having the same problems and unfortunately didn't read this page in time. I fixed the error by reformatting my time machine disk (off my airport extreme) and starting Time Machine from scratch - no errors now and everything is running smoothly. I lost all my old backups but hey that's life i suppose.
I have a related problem: I own an AP Extreme Base Station and have a harddisk attached to it. I use this etup as a file server for my workgroup. After several hours or days, the AP-attached harddisk refuses to have files saved/copied to it. Instead, whoever tries to write/copy something onto the disk gets an error message that the file name is to long or contains invalid characters. Copying files from the harddrive to an attached Mac does work, however. After a restart, everything is well again...
I will try to downgrade the software revision - but: does anyone have any suggestions what's the cause of this behavior?
Btw if anyone is interested: downgrading my airport base station to 7.4.1 solved the issue for me. With 7.4.2 there were intermittent errors where I could not write files to the harddrive attahced to the airport base station. The error was something along the lines of "filename too long or containing invalid characters". The error message did not appear again since downgrading.