Solved! At first I had not found other threads in the forum to fix the problem, but instead now i found this thread: Blank page after local install (edit: solved).
My case is exactly the same, a permission misleading. I had to add the owner ‘apache’ to all files in the grav directory.
The command (in Centos7):
ps aux | grep -v root | grep apache | cut -d. -f1 | sort | uniq
gave me the result ‘apache’, instead the grav directory owner was ‘root’.
Then, finally solved