Happy new year to you both, in addition to all, Ken, Mary, Visvanath, Gareth, Richard, Howard, Emma, Helen and all the names I missed!
May you all have successful and happy new year ahead.
Though Howard has already clearly mentioned not to use Oracle, for AFAIK, even Moodle core developers had given up on Oracle, and as far as more scalable and secure is concerned ,I have seen moodle installations serving more than a million users on MySQL.
BTW, you dont need a cluster for 100,000 users, if done right (in terms of hardware and software), a single box can handle 500,000 plus registered users.
Have you given right permission to IUSR to the moodledata folder?
as far as PHP manager is concerned, If you had properly read the post I linked, there are two more versions linked for Server 2012 and Server 2016, which clearly you've missed to say PHP Manager is incompatible. if, you already have settings in FastCGI, remove them before installing PHP manager as it will give error in presence of settings.
OK its FastCGI error, means something went wrong in setting up PHP.
you have to start again, removing fastCGI settings in IIS settings. have you been using PHP manager to setup PHP?
Daniel, this cant be possible that IIS blocks clients for Web Servers are built to serve public and as long as your clients are accessing the server through port 80, they shouldn't be blocked.
Have you made any changes to wwwroot folder regarding access permissions?
if in doubt, I have a tutorial up for setting up moodle on IIS, unless you have been through that already, I will recommend giving it a try and see what different you have done in setting up server and moodle, you can find the tutorial HERE.