max_children is quite low at 5

[krissrex]

The fpm www pool has pm.max_children = 5 which is easily reached by a single page load.
This can be seen in the logs (docker logs matomo_app_1): [01-Dec-2020 19:59:29] WARNING: [pool www] server reached pm.max_children setting (5), consider raising it

The config is located in /usr/local/etc/php-fpm.d/www.conf.
The setting is pm.max_children = 5 on line 114.

I arbitrarily set it to 20 and it works fine for now.

[J0WI]

You can easily mount your own config into /usr/local/etc/php-fpm.d/. There is no default that works for everyone.

[krissrex]

You can easily mount your own config into /usr/local/etc/php-fpm.d/. There is no default that works for everyone.

Understandable. However, in this case, the default doesn't "work" for anyone at all, it seems. A single dashboard load will trigger the problem.

[J0WI]

see also docker-library/php#1328