After some tests with a colleague with whom I share a CC folder (and subfolders), it appears that the files actually DO synchronize, both ways. The error message is about a partial failure, but I don't know which part fails. I'd expect that the folders successfuly synchronized have a green icon, and the failed ones have a red icon, but there are no more icons at all.
sync problems can occur because of a cc desktop app problem or a corrupt file (that's unable to sync) problem. to find the corrupt file (if there is one) requires trial and error requiring ~n steps for 2**n files (ie, ~10 steps for 1000 files).
if you want to pursue that and need help, let me know.
if it's a cc desktop app problem, it may resolve itself with the next cc desktop app update.
if don't want to wait for a cc update, let me know.