ROMManager does not backup my sd-ext
A possible solution is described e.g. in this modaco forum thread. In case of that user, the sd-ext partition got corrupted as it was mounted wrongly by the Android system (he used GPartEd to create his partitions which, regardless of the filesystem created (ext2/3/4, fat32), then were mounted as ext4).

So what he basically did: He factory reset the phone, deleted the partitions on SDCard, recreated them on phone (i.e. not with the card mounted to any PC, but directly on the phone, using CWMs options as far as I understood).

I didn't try that solution myself, as I'm not affected by the problem.