SOME blog posts not showing

I went to my host, presented the problem. They found the answer in one of your forums an fixed the problem. Now I’m having another problem and I don’t know if it because of the “fix” they applied or if it is something new.

The new issue is that when I am in WP, using the Avia advanced editor, when I go to “publish,” sometimes it doesn’t save and just gives me an error like this:
Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 14 bytes) in /home/magnific/public_html/wp-includes/wp-db.php on line 1822. Sometimes if I refresh that page with the error, it will give me the dashboard again.
Sometimes it saves, sometimes it won’t, but it will dump me back on the “pages” page or the “posts” page and not back on the page I’ve been editing.

Below are all the details from my correspondence with my web hosting service:
ME:
Some of my blog posts are not showing up on http://www.magnificentu.com. I’ve been in contact with the designer of the theme I’m using for wordpress … because I thought that was the problem. This is the response they sent:
“We get this error in the console when we visited one of the posts:
Failed to load resource: the server responded with a status of 500 (Internal Server Error) http://www.magnificentu.com/all-spiritual-paths-lead-to-love/
You should contact your hosting provider for more info regarding this issue.”

THEM:
Unfortunately I am not able to reproduce the issue from my side at this point. Mentioned link is opening properly, as well as other links I’ve checked on your website.

Admin area dashboard is loading with no problems as well:
http://www.magnificentu.com/wp-login.php

In order for us to proceed with the investigation, please provide the exact instructions to follow in order to reproduce the problem from our side. We are looking forward to hearing from you in the nearest time.

ME:
When I click on the blog, I see the various posts.

However, when I click on the first blog post, it does not render. It
doesn’t even put up the side bar.

It is only doing this with some of the posts, not all of them. I thought
this might be a theme issue, so I contacted the theme developer. It is
Enfold for wordpress. Anyway, the message I previously sent is what they
said to me.

I had given them login information for my wordpress dashboard so they could
try to adjust things in the theme. They have changed code for me previously
and fixed things with no problem. They got the server issue.

Let me know if you need the login info for wordpress.

THEM:
I have looked into the issue with http://www.magnificentu.com/all-spiritual-paths-lead-to-love/ post and after I have enabled WP_DEBUG at wp-config.php file, I have found the following error there:

Fatal error: Call to undefined function mb_strimwidth() in /home/magnific/public_html/wp-content/themes/enfold/framework/php/function-set-avia-backend.php on line 663

According to the recommendations at https://kriesi.at/support/topic/fatal-error-in-function-set-avia-backend-php/ I have enabled Mbstrings on the server and it resolved the issue. Could you recheck it from your side and let me know if any further assistance required on this matter?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
END OF CONVERSATION

That change did in fact fix the issue of the blog posts not showing up.
However, now I have this new problem that I mentioned at the top of this comment.
I have already supplied my log in information in the first post of this thread.

THANKS SO MUCH FOR LOOKING INTO THIS.