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.

can you check that the directory exists?

For some reason the themes were available on the admin panel even though they didn't have an individual folder. The export-import corrected that.

We're using 3.17 version and got this error. Is there any solution to this issue?

Just upgrade to Version 3.17.7 and experiencing the same problem - 'You can't save changes because the theme directory is not writable.'

Has anybody found a fix for this?

Thanks

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.