First of all, why are your folders set to 775 in the first place if your server creates an error on this permission? In the new docs, we recommend using 755 and I don't see any reason why your server would set 775 by default, then create an error because of it, and recommend setting 755 instead.
https://www.photo.gallery/x3docs/setup/permissions/
We do NOT preset permissions on files- and folders in X3 in the zip file, and they are set to a default after you uploaded by FTP based on server settings (and possibly FTP settings). If your server is reporting an error with incorrect permissions
after "installation (its not really an installation process, but just an upload), then there are two possible reasons for this:
A) 775 is the default permissions for your uploaded folders, either set by server or your FTP application (unlikely). This would be a very poor decision from whoever has set that default, because basically 775 is quite insecure, and your server even creates errors because of it.
B) Someone or somehow, the permissions have been set to 775 on your uploaded folders by mistake, either when you previously setup X3 or its been set manually.
X3 is not capable of setting permissions to any folders anyway. What is the link to your new /check/ page?