Work rejected. Server says: Stale or alien proof of work

[Bitconian]

I'm using BitMinter and I got this message:

Work rejected. Server says: Stale or alien proof of work

what does it mean? I looked information and apparently is very common when you mine solo. But I don't.

I've been getting this message all day. Have I mine something?? Or just running the computer for nothing?

2012.10.06 [18:24]  Tahiti (#1) ran out of work. Idling...
2012.10.06 [18:24]  Warning - slow network or server. Request took 21 sec.
2012.10.06 [18:44]  Work rejected. Server says: Stale proof of work
2012.10.06 [19:05]  Work rejected. Server says: Stale proof of work

Thanks!

Edit: is it a problem of my computer or is a BitMinter's problem?