Failed to acquire SSL session cache lock Apache 2.4 after running for several days

[Muhammad Hernawan <mwanmedia@xxxxxxxxx>]

Hi all, yesterday i sent question about this but i see "INVALID" on my email, is it fail?, ill try to resend it.

I'm using apache 2.4.29 on Ubuntu 18.04. I get warning msg on my apache logs that says "Failed to acquire SSL session cache lock". Here are the logs:

[ssl:warn] [pid 6667:tid 140575428806400] (22)Invalid argument: AH02026: Failed to acquire SSL session cache lock
[ssl:warn] [pid 6667:tid 140575428806400] (22)Invalid argument: AH02027: Failed to release SSL session cache lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH01948: Failed to acquire OCSP ssl-stapling lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH01949: Failed to release OCSP ssl-stapling lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH01948: Failed to acquire OCSP ssl-stapling-refresh lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH01948: Failed to acquire OCSP ssl-stapling lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH01949: Failed to release OCSP ssl-stapling lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH01949: Failed to release OCSP ssl-stapling lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH01949: Failed to release OCSP ssl-stapling-refresh lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH02026: Failed to acquire SSL session cache lock
[ssl:warn] [pid 6666:tid 140575453984512] (22)Invalid argument: AH02027: Failed to release SSL session cache lock

This issue occurs after running for several days (1-3 days) but after i restart apache service there is no warning msg and then it will occur in several days again. I use http2 and ssl-stapling. When the warning message appears, my website is still online and still accessible. However, I want to know the cause of this warning message that I still don't know.

More info:
Website: https://www.apkgeny.com/
  Server version: Apache/2.4.29 (Ubuntu)
Server built:   2018-10-03T14:41:08
Server's Module Magic Number: 20120211:68
Server loaded:  APR 1.6.3, APR-UTIL 1.6.1
Compiled using: APR 1.6.3, APR-UTIL 1.6.1
Architecture:   64-bit
Server MPM:     event
  threaded:     yes (fixed thread count)
    forked:     yes (variable process count)
Server compiled with....
 -D APR_HAS_SENDFILE
 -D APR_HAS_MMAP
 -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
 -D APR_USE_SYSVSEM_SERIALIZE
 -D APR_USE_PTHREAD_SERIALIZE
 -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
 -D APR_HAS_OTHER_CHILD
 -D AP_HAVE_RELIABLE_PIPED_LOGS
 -D DYNAMIC_MODULE_LIMIT=256
 -D HTTPD_ROOT="/etc/apache2"
 -D SUEXEC_BIN="/usr/lib/apache2/suexec"
 -D DEFAULT_PIDLOG="/var/run/apache2.pid"
 -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
 -D DEFAULT_ERRORLOG="logs/error_log"
 -D AP_TYPES_CONFIG_FILE="mime.types"
 -D SERVER_CONFIG_FILE="apache2.conf"

Here is dump config apache2ctl -t -D DUMP_RUN_CFG :
ServerRoot: "/etc/apache2"
Main DocumentRoot: "/var/www/html"
Main ErrorLog: "/var/log/apache2/error.log"
Mutex ssl-stapling: using_defaults
Mutex proxy: using_defaults
Mutex ssl-cache: using_defaults
Mutex default: dir="/var/run/apache2/" mechanism=default
Mutex watchdog-callback: using_defaults
Mutex rewrite-map: using_defaults
Mutex ssl-stapling-refresh: using_defaults
PidFile: "/var/run/apache2/apache2.pid"
Define: DUMP_RUN_CFG
User: name="www-data" id=1000
Group: name="www-data" id=1000

I'd appreciate your help. Thanks in advance.