In order to solve this issue you should check the following:
Following files should have permissions set to 664 (-rw-rw-r--):
Please make sure that permissions for the following folders are set to 775 (drwxrwxr-x):
mt-content/assets/less ~all folders and files that there will be
mt-content/themes/~all folders and files that there will be
The rest of the files should have the permissions set to 644 (-rw-r--r--).
The rest of the folders should have the permissions set to 755 (drwxr-xr-x).