System's effected: Ubuntu 16.04, Ubuntu 18.04, not 14.04.
Frequency: Approx. around 24 - 36 hours.
[Wed May 08 03:44:25.964597 2019] [mpm_prefork:notice] [pid 7670] AH00171: Graceful restart requested, doing restart
[Wed May 08 03:44:26.173977 2019] [mpm_prefork:notice] [pid 7670] AH00163: Apache/2.4.39 (Ubuntu) OpenSSL/1.1.1b configured -- resuming normal operations
[Wed May 08 03:44:26.173993 2019] [core:notice] [pid 7670] AH00094: Command line: '/usr/sbin/apache2'
[Wed May 08 03:49:04.001821 2019] [mpm_prefork:notice] [pid 7670] AH00171: Graceful restart requested, doing restart
[Wed May 08 03:49:04.210024 2019] [mpm_prefork:notice] [pid 7670] AH00163: Apache/2.4.39 (Ubuntu) OpenSSL/1.1.1b configured -- resuming normal operations
[Wed May 08 03:49:04.210039 2019] [core:notice] [pid 7670] AH00094: Command line: '/usr/sbin/apache2'
[Wed May 08 03:54:39.080243 2019] [mpm_prefork:notice] [pid 7670] AH00171: Graceful restart requested, doing restart
[Wed May 08 03:54:39.298743 2019] [mpm_prefork:notice] [pid 7670] AH00163: Apache/2.4.39 (Ubuntu) OpenSSL/1.1.1b configured -- resuming normal operations
[Wed May 08 03:54:39.298757 2019] [core:notice] [pid 7670] AH00094: Command line: '/usr/sbin/apache2'
[Wed May 08 03:59:17.197985 2019] [mpm_prefork:notice] [pid 7670] AH00171: Graceful restart requested, doing restart
[Wed May 08 03:59:17.447167 2019] [mpm_prefork:notice] [pid 7670] AH00163: Apache/2.4.39 (Ubuntu) OpenSSL/1.1.1b configured -- resuming normal operations
[Wed May 08 03:59:17.447182 2019] [core:notice] [pid 7670] AH00094: Command line: '/usr/sbin/apache2'
[Wed May 08 04:03:55.095152 2019] [mpm_prefork:notice] [pid 7670] AH00171: Graceful restart requested, doing restart
libgomp: could not create thread pool destructor.
Manual restart of the apache service is needed after it has stopped.
Don't know why it involves libgomp
...
2017-07-30: Seems like it has something todo with the combination Apache - PHP - ImageMagick
Removing ImageMagick from Apache-PHP but keeping it in PHP-CLI removed the problem. But it's not fix...