JarnoG, so... I think I followed your instructions, but I am still having errors. See my process and some screenshots.
1. I turned off Transmission.
2. I set permissions in Frontview
3. (I then selected advanced, and ticked Set ownership and permission for existing files and folders in this share to the above settings. This option is useful in cases where you are changing security levels and need to workaround file access problems. Then pressed save)
4. I reloaded Transmission, and I get this error after about 200-500mb of downloading