最近，我的Ubuntu计算机上的Apache2在启动几个小时后终止，前提是没有人修改任何Apache配置或发出任何SIGTERM信号（apt / apt-get升级除外）。当我检查error.log时，它会显示类似这样的东西，我找不到任何线索：
[Mon Jun 04 00:05:36.567117 2018] [mpm_prefork:notice] [pid 2713] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:05:36.567139 2018] [core:notice] [pid 2713] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:08:42.692780 2018] [mpm_prefork:notice] [pid 2713] AH00169: caught SIGTERM, shutting down
[Mon Jun 04 00:08:49.095482 2018] [ssl:warn] [pid 8974] AH01906: 167381ca9f7090bf9edf80ba7abf2757.6b6358cf1d262e93db083c8b42c3e7e4.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:08:49.132763 2018] [ssl:warn] [pid 8975] AH01906: 167381ca9f7090bf9edf80ba7abf2757.6b6358cf1d262e93db083c8b42c3e7e4.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:08:49.138388 2018] [mpm_prefork:notice] [pid 8975] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:08:49.138411 2018] [core:notice] [pid 8975] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:08:55.724193 2018] [mpm_prefork:notice] [pid 8975] AH00171: Graceful restart requested, doing restart
[Mon Jun 04 00:08:55.761313 2018] [mpm_prefork:notice] [pid 8975] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:08:55.761330 2018] [core:notice] [pid 8975] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:09:00.420832 2018] [mpm_prefork:notice] [pid 8975] AH00171: Graceful restart requested, doing restart
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
[Mon Jun 04 00:09:00.454148 2018] [ssl:warn] [pid 8975] AH01906: 188454998ed00edb88cd5d07b2ab2254.3b3a904d9d9487cc6497b164e7f62a83.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:00.454369 2018] [ssl:warn] [pid 8975] AH01906: c2322864937b675b512cc9082dced726.92496ce8d61bce3f4a23bc1a3218e316.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:00.454577 2018] [ssl:warn] [pid 8975] AH01906: 76af4c76944afa8d20dc39e2298db3b3.e13999bec38b4f773ce0a42cd05bbd4c.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:00.454777 2018] [ssl:warn] [pid 8975] AH01906: 17692edb40ba2833effa12d8a5c88387.667c0669638111d9584e8e5e06ff19b9.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:00.454982 2018] [ssl:warn] [pid 8975] AH01906: ae632be6f870530686007c705e552d70.a1bcbaaf18efb59cf234338286ca537b.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:00.455186 2018] [ssl:warn] [pid 8975] AH01906: 953872e180e5982e31690363dd4a30d8.6ca1b4a315f407a052938df2d18acba3.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:00.455397 2018] [ssl:warn] [pid 8975] AH01906: 6583ed0337c393064b82f1e8123809ba.e6548597537804f2bef7369a4c2acdba.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:00.458601 2018] [mpm_prefork:notice] [pid 8975] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:09:00.458610 2018] [core:notice] [pid 8975] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:09:10.021985 2018] [mpm_prefork:notice] [pid 8975] AH00171: Graceful restart requested, doing restart
[Mon Jun 04 00:09:10.061542 2018] [mpm_prefork:notice] [pid 8975] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:09:10.061567 2018] [core:notice] [pid 8975] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:09:12.672320 2018] [mpm_prefork:notice] [pid 8975] AH00171: Graceful restart requested, doing restart
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
[Mon Jun 04 00:09:12.707771 2018] [ssl:warn] [pid 8975] AH01906: 074653103fcd03ca40aa6ae4b90266f9.e453fad80449cbedfb50933e788548a7.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:12.708001 2018] [ssl:warn] [pid 8975] AH01906: 4ac70f37b3cc16c2eedecc274ada88b5.fc9272a4d656a16b16687b53e5ddab16.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:12.711091 2018] [mpm_prefork:notice] [pid 8975] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:09:12.711098 2018] [core:notice] [pid 8975] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:09:19.800865 2018] [mpm_prefork:notice] [pid 8975] AH00171: Graceful restart requested, doing restart
[Mon Jun 04 00:09:19.839602 2018] [mpm_prefork:notice] [pid 8975] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:09:19.839618 2018] [core:notice] [pid 8975] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:09:22.113155 2018] [mpm_prefork:notice] [pid 8975] AH00171: Graceful restart requested, doing restart
AH00112: Warning: DocumentRoot [/var/lib/letsencrypt/tls_sni_01_page/] does not exist
[Mon Jun 04 00:09:22.149880 2018] [ssl:warn] [pid 8975] AH01906: 95a5f7529d708ef2ed9efc1770f7b3f9.7d2f8675f76c8b2e6909bbe900e67d93.acme.invalid:443:0 server certificate is a CA certificate (BasicConstr$
[Mon Jun 04 00:09:22.153017 2018] [mpm_prefork:notice] [pid 8975] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:09:22.153025 2018] [core:notice] [pid 8975] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:09:28.755711 2018] [mpm_prefork:notice] [pid 8975] AH00171: Graceful restart requested, doing restart
[Mon Jun 04 00:09:28.798520 2018] [mpm_prefork:notice] [pid 8975] AH00163: Apache/2.4.27 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Mon Jun 04 00:09:28.798537 2018] [core:notice] [pid 8975] AH00094: Command line: '/usr/sbin/apache2'
[Mon Jun 04 00:09:28.975472 2018] [mpm_prefork:notice] [pid 8975] AH00169: caught SIGTERM, shutting down
任何人都可以提供帮助？谢谢！