E' un bug di Ubuntu One, questo mi hanno risposto da Ubuntu support (peraltro molto efficienti nel rispondere):

Hi,

Sorry for the problems. We don't recommend syncing folders like the
.evolution folder since it normally contains many files (some of them
quite large) that change often.

The conflicts happen when the client cannot tell which version is
correct - the client or the server. This can happen on Ubuntu 10.04 in
some cases where it shouldn't. We're working on backporting the fixes
we've made in newer versions of the Ubuntu One client to the version in
10.04.

Thanks,

Joshua Hoover