LiveG

These are all saying your PHP-FPM does not have enough processes to handle your traffic.  You should look at optimizing your PHP-FPM configuration.   I would also suggest looking at the number of Apache processes you have.  It MAY be an attack, but if this has been going on for a month and a half, it's not very likely.  They don't typically target sites that long unless they're looking to get something from you.  
It sounds more like you need to optimize your system resources more or move to a bigger server and optimize it.  

Test your security headers
https://securityheaders.com/
and show your results

By the way...  if you don't know where your FPM config is and you have root access.... you can run the following command typically:
ps aux | grep php-fpm| grep master In that case, the config file that FPM is loading from would be /usr/local/etc/php-fpm.conf.  You could edit the file in a text editor such as pico, nano, or vim.  The file could also be just /etc/php-fpm.conf, or be in a bunch of other places.  That's why it's important to know your specific server setup/config.  

In the errors described by the OP where he's copy/pasted an IPB trace, each one has had 3rd party code or resources.  He now says each 3rd party resource is disabled and now he's running out of system resources.  
In addition, he's posted the following as multiple entries from the PHP log:
Having a poorly configured php-fpm configuration CAN lead to system stability issues when the max number of instances that are able to be handled by PHP is reached and it can lead to a dramatic increase in the system load average.  Thus the suggestion to optimize the config and/or increase system resources.  
I'm not sure how checking an end user's security headers presented will change system utilization or diagnose code level issues.  So I've intentionally not commented on the suggestion you've presented.  I assume you have an idea about something outside of it being a configuration issue or IPB code or 3rd party resource issue that I've not considered.  (Which is great...  the more ideas to try the better!)  

You can generally tell when there is a big issue with the software because dozens of people are here with the same issue.  This topic was created a month and a half ago and there have not been any type of swells of "me too" type messages.   So it's likely not a basic code issue that is impacting the majority of users.  Now... there could be a problem with how your specific server configuration is setup with the latest version of the software, but that would require you to dig into your server logs to understand that.  
.GZ are like ZIP files.  They're compressed files that contain data. So no, they're not real time.  At the end of the day, you need to figure out where or even if your server logs PHP errors to and review those.  You theoretically could watch those files in real time if you knew where they were.  We can't tell you where those files are because there is no one specific spot for them.  It depends on how your specific server was setup.  Different OS put files in different places and if you use a control panel, it could be somewhere else even different from the OS default.  You may need to hire some sort of server administrator to dig into your server configuration for you if you are not familiar with getting the details of your specific setup.  
Also...  if your board works for a bit and then a few hours later stops working, it's possible your server can't handle the volume of traffic and either needs more resources or needs the server configuration to be optimized.  (For example, improving the mySQL my.cnf file, or optimizing the number of Apache processes for the amount of resources of the server.)  
 

There are enough descriptions of Apache 2 compatibility with Linux - read and review what is not compatible with your installation. I asked for the contents of two files to you show them, not to confirm their existence... It is possible that some rule in these files is creating an issue that is stressing you out.

Try and test your website for viruses, for example, with this online tool
https://online.drweb.com/result2/?url=1

Restore that template bit. Should help (as it will remove hasMarkers part).

Thank you.
I will address the author of the team!
 

It just means that in a used theme so far, there are outdated codes, or there are codes that create a conflict. Search for an author of your theme to provide you with an update on it.

There's no hasMarkers method in our code, sounds like a very old template.
Can you compare your forums/front/global/row template with the default one?

When you are able to log in to ACP, switch the original IPS theme to the default theme.

Is that custom theme compatible with your forum version? if not it needs to be upgraded or uninstalled. 

Forums automatically prevent consecutive replies within a few seconds but I don’t think this time is configurable.
If you don’t want the same member to post consecutively, regardless of the time, you will need a custom application.

Feel free to request it in the feedback forum.  
 
The merge on additional reply feature was designed specifically for this so they would not have to block someone from contributing if they forgot something important.  
Personally while it may make sense for your community, I’m not sure it does for everyone.  The merge approach is much more user friendly and does not stop someone who presses submit too quickly by accident.  But that’s just my two cents. So suggest away. 🙂 

Upgraded and deactivated this option as recommended. i will monitor it.
Thank you @Jim M

You can use Pages or write a simple app with one controller that will show a page with an iframe, like this:

Update:
I have succeeded in solving the second issue ( need to configure the system to show it in independent mode)
I still have has the notification issue.

By using page like this:
https://invisioncommunity.com/attachments/
Location:

That forum is contained within another forum above it, of which has no permission to view for guests. So they cannot see anything within 
Im not sure where you believe you had sent the pm 10 times there. I have received only one.

10x is a shortcut of thanks 🙂
It works, thank you for the great assistant! @Marc Stridgen

Hi @MythonPonty
There's already a feature for adding a counter badge. 👍

Invision Community offers fantastic ways of customizing the user experience for your members, and today, we’re excited to introduce some new and really simple ways of customizing Invision Community 5 even further using our new icon tools.
 
Icon Picker
Lets begin with our brand new icon picker. Containing both Font Awesome icons and emojis, the new picker allows you to easily search and assign icons to specific areas throughout your site. Lets take a look at some examples!
icon-picker.mp4  
Navigation icons
Adding icons to the navigation list has been a highly requested feature, so we're happy to announce that you can now use this new picker to do exactly that, for both the horizontal and vertical navigation panels, without needing to modify your theme.

 
Forum icons
Uploading forum icons is a great way to personalize individual areas of your community. In the past, these icons have typically been images, uploaded via the admin panel. In addition to the upload form, the icon picker now makes it a breeze to assign icons to forums - and if a Font Awesome icon is chosen, it'll even inherit the featured forum color.

 
Forum Feature Color
We have brought the existing forum feature color to feed view allowing for a flash of color and personalization that helps associate a color with a specific forum. The feature color pairs really well with the card image to lift the forum display.

 
Icon creator for badges, ranks and reactions
Creating unique badges, ranks and reactions is a great way to boost activity within your community by encouraging members to share more engaging and frequent content - but designing these icons from scratch using a graphics program often comes with hurdles of its own.
With our new icon creator, you can now design your own custom icons for badges, ranks and reactions straight from your Admin panel, using a combination of colors, icons and shapes.
icon-creator.mp4  
 
We think this new icon creator will make the rank, badges and reactions features even more accessible for everyone, allowing you to create a user experience that is uniquely yours. With Invision Community 5, bringing in customization and personalization moves beyond adding new themes.
We're excited to see how you can take advantage of these new tools, and we look forward hearing your feedback in the comments below!
 


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Take a look, i think you would like the style
http://www.emoticons.online.fr/

I think this means you want to have the editor hidden. If so, custom CSS:
li#_report_message{ display:none; }