I found the solution (at least to my problem with the 0 kb files) thanks to this page.
Go to Settings --> Apps, choose in the three dot menu “Show system”, and delete data and cache for the apps “External Storage” and “Media Storage”. Then reboot the phone.
The file index will be rebuild and everything works fine again 
Edit: Permissions to store files on the SD card or to edit them need to be given again to the camera and gallery apps.