i3itCoin removing the .htaccess will obviously result in a 404 because the url won't be redirected to the application anymore. Assuming it's therefor a htaccess issue is incorrect, 500 errors can be throw by both the application and the webserver. In order to identify which is causing the error you have to look at the response headers of the call made. Luckily you posted your installation link, so I could confirm the issue lies with the webserver:
Connection:close
Content-Length:679
Content-Type:text/html; charset=iso-8859-1
Date:Thu, 28 Apr 2016 06:20:17 GMT
Server:Apache/2.4.18
In case the application would have thrown the error, PHP would have been mentioned in here somewhere.
Now that we've established that, looking at your htaccess and running it through an online validator, it works. However I did copy paste the file from your snippet and it added a "view this snippet online in pastebin" at the top. Might you have copied that same line to your flarum installation?
If that does not work you do have to check the error log of apache for either this website or for the server in general to identify the cause as @zegaroid correctly pointed out already.