Unison persistently refused to synchronise new files not present on one of the machines claiming:
"Failed to set permissions of file …" (different user names and IDs, by the way).
The solution (found here) is to disable owner and permission setting in the corresponding ~/.unison/<theproject>.prf file. Add the lines
and you should be fine.
Update (12/2014): I ran into a permissions problem again after installing a fresh 14.10 Kubuntu while keeping my old home partition (at least I think that's what changed between Unison runs ;) ).
This time it was "Error in setting permissions". This bug discussion recommended adding a
line to the .prf file (in addition to the options listed above), which solved my issue.