It does not have to do with one person or windows/mac/linux os.
It has to do with the following Goddady certificate
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If a user/system has already opened a web page with that certificate the download will proceed and the game will launch normally.
If a user/system has never encountered a page signed with that certificate unity will stop the download since it does not find the certificate in the system and considers the connection insecure.
Bellow I attach in the file "Godday cert.zip", the reg key of the certificate. Extract and doubleclick the file "Godday cert.reg" to import it in your system certificates. (= No more blackscreens with the spellbook)
edit: firefox does not use the system's certificate store and this is the reason it fails for firefox windows users
more info here
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przewodo you may want to add this to the faq page...