If you're up for a self-hosted option, Seafile is great. The server and the client are both pretty lightweight. You should create and store encrypted volumes yourself and not trust its encryption mechanism, but it handles delta sync very well, which means it's only sending the pieces that change (and e.g. a Veracrypt/Truecrypt volume doesn't change a lot when adding/removing data from a volume, so you won't sync a lot for example with OwnCloud, which also has the nasty habit of eating your files).