The latest stable patch (r12158) includes changes to the way fleet data is stored -- and these changes are not backwards compatible.
This is the first true "breaking change" I've posted since beta started -- in the past, older saves might be bug-prone, but, they'd still often work. That's not the case with this patch, however, which means that you'll no longer be able to load any saves stamped with version numbers lower than r12142.
Sorry if this disrupts any ongoing galactic wars