I'm trying to backup my home directory (excluding a few folders). The backup is saved on a ~300 GB partition (called backups) of an internal drive, but not the same drive as the home directory. When I try to backup, everything goes OK until at some point Déjà Dup says there is no space left on "backups". The problem is that Déjà Dup only used ~20 GB at this point, so there is a ton of space left. I tried several times without luck.

Why is Déjà Dup saying this?

The size of my home directory, without the excluded folders, is 136 GB.