apache2 crashes suddenly and leaves zombie processes

I am experiencing recurring problem with my apache server, where after few hours/days of light usage, the apache process suddenly crashes and leaves zombie processes. In ps, I then see this: root 1209 ? Ss 0.0 1.3 Aug04 /usr/sbin/apache2 -k start www-data 15531 ? Z 0.0 0.0 Aug15 [apache2] www-data 15539 ? Z 0.0 0.0 Aug15 [apache2] www-data 15540 ? Z 0.0 0.0 Aug15 [apache2] www-data 15541 ? Z 0.0 0.0 Aug15 [apache2] www-data 15542 ? Z 0.0 0.0 Aug15 [apache2] www-data 15543 ? Z 0.0 0.0 Aug15 [apache2] www-data 15544 ? Z 0.0 0.0 Aug15 [apache2] www-data 15545 ? Z 0.0 0.0 Aug15 [apache2] www-data 15546 ? Z 0.0 0.0 Aug15 [apache2] www-data 15547 ? Z 0.0 0.0 Aug15 [apache2] www-data 15548 ? Z 0.0 0.0 Aug15 [apache2] www-data 15549 ? Z 0.0 0.0 Aug15 [apache2] www-data 15550 ? Z 0.0 0.0 Aug15 [apache2] www-data 15551 ? Z 0.0 0.0 Aug15 [apache2] www-data 15552 ? Z 0.0 0.0 Aug15 [apache2] www-data 15553 ? Z 0.0 0.0 Aug15 [apache2] www-data 15554 ? Z 0.0 0.0 Aug15 [apache2] My Apache server has very light use, I can browse the server fine for a while, but then I click a link and it suddenly crashes. I am not able to pinpoint what exactly causes the crash, seems random to me. My server is Debian Buster, and the apache server is using php5 framework (for Dokuwiki). Following packages are installed: apache2 2.4.38-3 apache2-bin 2.4.38-3 apache2-data 2.4.38-3 apache2-utils 2.4.38-3 libapache2-mod-php5.6 5.6.40 php5.6 5.6.40 php5.6-cli 5.6.40 php5.6-common 5.6.40 php5.6-curl 5.6.40 php5.6-json 5.6.40 php5.6-mysql 5.6.40 php5.6-opcache 5.6.40 php5.6-readline 5.6.40 php5.6-sqlite3 5.6.40 php5.6-xml 5.6.40 **How can I troubleshoot this problem?** There is no message in any of my logs (dmesg, syslog, or apache errror log)