when you are already forcing httpS
then you can correct this to be https
so no time is wasted redirecting to https and for additional REQs
do you have any .htaccess file on the subdomain?
and of course do from the post above by @Greenreader9
and for error pages you have to write a valid html document (but progress can be seen )
copy/paste code and validate here The W3C Markup Validation Service
For now I just made another subdomain with InfinityFree without SSL. (http://hosting.rydercragie.com). Facebook is fine with that one so I’ll use that instead for now.
try to edit manually and not via the control panel
that your .htaccess file where the paths for error pages are listed
if this above is not the cause of the problem:
You probably have a set of different problems and not just one
I don’t know why the httpS URL version of the error page would lead to 404
except that I mentioned before that you have something that hides extensions
and then something in .httacess that causes it not to be able to find the file after it chews all the code.
Perhaps the sequence of actions in .htaccess is also misaligned
you need to approach it methodically
turn off all .httaccess (or code sections by adding # in front)
and see where the problem is.
or clearly state the extension for each file
and so piece by piece in small steps until you find the culprits
and I have the impression that you do not want to show us any URL from a files that you want to share on FB or TW probably because you are violating TOS