First a few security things to keep you safer. When you post stuff like that, always remove your ip and also your username from the display and any other data that is personal to the server, noone needs that info to help you. We just need the error like so:
[Sat Jan 07 08:46:52 2017] [error] [client xxxxxx] File does not exist: /home/xxxxxx/public_html/404.shtml, referer:
we dont need any more than that, so you might want to reformat that page and remove it and repost it.
Getting rid of half your issue is easy. Just create a 404.shtml file in your domain root. Just create it just like you would a normal 404.html file but name it 404.shtml you can leave it blank if you want or put some text in there as you would a normal 404 file, thats up to you. That will solve half your issue and get rid of that one. Let me look at the other one and see what can be done here.
Also pay attention more to the error log file that may show up in your domain root or other folder rather so much on cpanel error log, they do work differently. That does not mean that you dont need to pay attention to cpanel error log, you can get good feedback if someone is trying to hack your site with repeated requests and such but just use that as a feedback mostly rather than actual fatal errors. I would use the error log file in your root for that, it may not be there until you actually have an error.
back to the htaccess question?
Does your server meet all the specs on the RIGHT SIDE of this page. http://www.oxwall.org/hosting
Also try running my free checker to check the same thing, if your on shared hosting they probably have shell turned off so you wont get all of the report but youll get quite a bit.. Run this plugin and let me know the outcome please.
You will have to check some of that requirements list on the page manually because your on shared, or ask your host.
https://developers.oxwall.com/store/item/1356
The things with the red does not mean your necessarily missing them, its that shell exec has been disabled and some of those values the only way to check is via shell exec or manually in the server config. So im not so worried about those, just make sure that your server meets the requirements that chris posted.
So here is what you do, you know how to create a file now in file manager or you can just add it to the config file, so just follow this guide, then let us know what your real php version is.
be sure you read it over first so you can decide which way you want to do it.
https://developers.oxwall.com/forum/topic/47677