In order to be detected the directories containing addons, including the base ones, must have the write flag enabled.
You can have files without the write permission in the said directories but the directory itself needs that write flag.
It does not seems logic since the game wants to READ the addons. Maybe the directories are opened in read-write ?
This is a problem when you want to setup multiple servers with one installation and symbolic links.
I already foud a workaround for it but it does not seems logic anyway.