Dear All,
I've done a fair amount of digging and I've not found anything that can help me on this one.
I've setup a wallet on my database and imported the following certificates.

When I run:
SELECT utl_http.request('https://accounts.google.com/o/oauth2/auth',null,'file:C:\OracleDatabase\product\12.1.0\dbhome_1\owm\wallets\oracle','superman99') from dual;
I get:
ORA-29273: HTTP request failed
ORA-29024: Certificate validation failure
ORA-06512: at "SYS.UTL_HTTP", line 1491
ORA-06512: at line 1
Running it for https://google.com is successful. even running it for https://maps.google.com is successful.
Doing for further digging around I notice that there is a call to the following:

Naviaging to this i find a *.google.com certificate:

I've tried exporting and loading this certficate several times in severeal ways and in different formats but I always get the same exception:

I have tried removing all certificates except this one and I still get the same error.
My Oracle version is:12.1.0.2.0 Enterprise Edition
On: Microsoft Windows x86 64-bit
Update I found the following article, and I've tried removing the endpoint URL SSL certificate, but sadly it's still not working:
Thanks for your help!
Peter