After updating to 1.4.91 in both machines, and stopping and restarint the sync several times, it seems I've get rid of the problem. The thing is that I keep the 'Always check for updates to this version' in the advanced preferences, and it never was aware of a new version, no even when clicking in the 'Check now' button. Is that a bug? It is certain that 1.4 feels a bit unreliable (at least until now), but to me it has been a great update, especially regarding to the use of CPU resources (and then, to the impact on battery life for laptops).