Issue Cannot allocate memory

[cpt.viktor]

Hi Guys,
i have my first big problem, cant find the correct anwser.
So I hope you have the time to help me.

After a while some services shutdown because of "Cannot allocate memory ".

This was my first Error Message:

Can not restart web server: INFO: [Mon Dec 16 14:06:39 CET 2019]: Service: httpd, Action: start Trying to start service httpd... failed Dec 16 14:06:27 xyz-server.com systemd[1]: Failed to start The Apache HTTP Server. Dec 16 14:06:27 xyz-server.com systemd[1]: httpd.service failed. Dec 16 14:06:37 xyz-server.com systemd[1]: Failed to fork: Cannot allocate memory Dec 16 14:06:37 xyz-server.com systemd[1]: httpd.service failed to run 'start' task: Cannot allocate memory Dec 16 14:06:37 xyz-server.com systemd[1]: Failed to start The Apache HTTP Server. Dec 16 14:06:37 xyz-server.com systemd[1]: httpd.service failed. Dec 16 14:06:41 xyz-server.com systemd[1]: Failed to fork: Cannot allocate memory Dec 16 14:06:41 xyz-server.com systemd[1]: httpd.service failed to run 'start' task: Cannot allocate memory Dec 16 14:06:41 xyz-server.com systemd[1]: Failed to start The Apache HTTP Server. Dec 16 14:06:41 xyz-server.com systemd[1]: httpd.service failed. ***** problem report ***** Warning: start service httpd failed /usr/local/psa/admin/sbin/pleskrc execution failed: Job for httpd.service failed because a configured resource limit was exceeded. See "systemctl status httpd.service" and "journalctl -xe" for details.

I checked the status with "systemctl status httpd.service" and "journalctl -xe":

[root@7-server ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
  Drop-In: /usr/lib/systemd/system/httpd.service.d
           └─limit_nofile.conf
   Active: active (running) since Di 2019-12-17 15:41:16 CET; 18min ago
     Docs: man:httpd(8)
           man:apachectl(8)
 Main PID: 7313 (httpd)
   Status: "Total requests: 83; Current requests/sec: 0; Current traffic:   0 B/sec"
   CGroup: /system.slice/httpd.service
           ├─7313 /usr/sbin/httpd -DFOREGROUND
           ├─7315 /usr/sbin/httpd -DFOREGROUND
           ├─7316 /usr/sbin/httpd -DFOREGROUND
           ├─7317 /usr/sbin/httpd -DFOREGROUND
           ├─7318 /usr/sbin/httpd -DFOREGROUND
           ├─7319 /usr/sbin/httpd -DFOREGROUND
           └─7892 /usr/sbin/httpd -DFOREGROUND

Dez 17 15:41:14 xyz-server.com systemd[1]: Starting The Apache HTTP Server...
Dez 17 15:41:14 xyz-server.com httpd[7313]: [Tue Dec 17 15:41:14.744112 2019] [so:warn] [pid 7313] AH01574: module actions_mod...kippingDez 17 15:41:14 xyz-server.com httpd[7313]: [Tue Dec 17 15:41:14.843360 2019] [so:warn] [pid 7313] AH01574: module headers_mod...kippingDez 17 15:41:14 xyz-server.com httpd[7313]: [Tue Dec 17 15:41:14.862180 2019] [so:warn] [pid 7313] AH01574: module logio_modul...kippingDez 17 15:41:14 xyz-server.com httpd[7313]: [Tue Dec 17 15:41:14.892360 2019] [so:warn] [pid 7313] AH01574: module suexec_modu...kippingDez 17 15:41:16 xyz-server.com systemd[1]: Started The Apache HTTP Server.
Hint: Some lines were ellipsized, use -l to show in full.

This is my Server Memory with "free -h":

                       total        used        free      shared  buff/cache   available
Mem:           3,6G        1,0G        367M        334M        2,3G        2,1G
Swap:          7,6G        5,2G        2,4G

And this is my Plesk Memory Usage Output, from "Tools -> Server Informations":

Memory Usage

Total
Used
Free
Shared
Buffer
Cached
Usage
3.65 GB
3.31 GB
349.15 MB
281.99 MB
301.90 MB
2.05 GB
96.65%

Swap Usage

Total
Used
Free
Usage
7.63 GB
5.22 GB
2.41 GB
68.42%

I dont know why my Plesk Memory Usage is constantly high (90-97%) and how to clean it up.
I tried to deactive some vhosts and removed Databases, kill Processes, uninstall useless Server & Plesk Addons.
Server worked perfect for 644 Days, with weekly maintenance. Now iam confused a little bit about this situation.

kind regards
thanks you =)