It’s either simply the permission, or there is a firewall (or malware scanner) active, which makes the files unaccessible. In that case if you change the permission, the file will be still unaccessible. Last time it was a firewall, it suspected that file as a false-positive and blocked it.
It can be a security related plugin, or firewall/scanner at your hosting provider I suppose.
Best,Ernest Marcinko
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