When I put online-mode false in server.properties, every time that I restart the server all clans appear without members.

I think it´s about uuids and stuff of Mojang.

I´m using Spigot 1.8.3

Is there any solution?

Thanks for all.

Tough one, UUIDs change when offline/online. This would require saving both, thus a major change on the way things are currently architected. The cost/reward for this is pretty low, not many people switch back and forth. Will probably not fix this.