I've just done a clean install (apart from the DB which was updated automatically) of version 3.14.8+180829 and I have the same issue.
I have given all rights to everyone (777) on the folders recursively and still the message is there.
Has there been a breakthrough ?
Pre-posting edit: actually managed to correct the issue by exporting and re-importing the theme.
ComfortUpdate does not change directories permissions, so the problem is not related to your update. I'd suggest you check the permissions on the upload directories.