The screenshots may suggest an attack, however you would need to further investigate to understand and resolve the issue.
If it is the session table in your screenshot, it appears to show a small number of IPs. It is possible that it could be that you have another service accessing the CMS, causing the high load in your screenshots.
You may want to look at your access logs to see if you can narrow down where the issue is coming from. If you are using a reverse proxy, then those logs would be a good place to start.
I cannot provide any further support on how to secure your web service but I do hope that you are able to locate the problem and resolve it.
Last question : what does it mean if in the active column there is the “check” sign and if the user column is empty? Is the “hacker” logged on the CMS?
Just loading the login page triggers almost no load on the CMS.
If that’s taking your server down, then I would suggest it’s either under specified, or the amount of traffic you’re receiving is way outside what is in the screenshots (which is a few connections only).
In that case you’d need to be using something like Cloudflare (which I’ve not tested) to front the CMS.
I’m not sure what you mean by a redirection service, but the Players make POST requests so won’t be able to work if there’s any kind of HTTP redirect in their path.