I'm assuming the php in Kimai is not reading or is overriding the /app/data/php.ini file, from the reply I got from the Kimai dev, and with further testing that didn't change the value in the error messages.

Would you know where the php memory limit is being set to 64M, if that's the case? Could Cloudron be messing with that value?

Kimai has a 2 GB memory limit currently.

I added that line to the php.ini in Kimai, and rebooted Kimai from the Cloudron panel. I then did the same again with 256M. I got the same error each time:

[2022-01-07 17:09:48] php.CRITICAL: Fatal Error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 2105344 bytes) {"exception":"[object] (Symfony\Component\ErrorHandler\Error\OutOfMemoryError(code: 0): Error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 2105344 bytes) at /run/kimai/cache/prod/twig/1f/1f96a1be1b05a205498d597983e88903dced893044df82d2d3324d14269c59ff.php:1203)"} []
[2022-01-07 17:09:48] request.CRITICAL: Uncaught PHP Exception Symfony\Component\ErrorHandler\Error\OutOfMemoryError: "Error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 2105344 bytes)" at /run/kimai/cache/prod/twig/1f/1f96a1be1b05a205498d597983e88903dced893044df82d2d3324d14269c59ff.php line 1203 {"exception":"[object] (Symfony\Component\ErrorHandler\Error\OutOfMemoryError(code: 0): Error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 2105344 bytes) at /run/kimai/cache/prod/twig/1f/1f96a1be1b05a205498d597983e88903dced893044df82d2d3324d14269c59ff.php:1203)"} []

Is it saying php only has 67108864 bytes available? If so, Am I missing a step in getting it to read the php.ini?

memory_limit = 128M

and also make sure the app as such has the limit higher than that.

I have increased the memory limit since then, but it has not seemed to help. I'll post the Kimai prod.log entries here as well, in case it helps:

[2022-01-07 13:21:06] request.INFO: Matched route "quick_entry". {"route":"quick_entry","route_parameters":{"_route":"quick_entry","begin":null,"_controller":"App\Controller\QuickEntryController::quickEntry","_locale":"en"},"request_uri":"https://my.kimai.com/en/quick_entry","method":"GET"} []
[2022-01-07 13:21:06] security.DEBUG: Read existing security token from the session. {"key":"_security_secured_area","token_class":"Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken"} []
[2022-01-07 13:21:06] security.DEBUG: User was reloaded from a user provider. {"provider":"App\Security\KimaiUserProvider","username":"person"} []
[2022-01-07 13:21:06] security.DEBUG: Checking for guard authentication credentials. {"firewall_key":"secured_area","authenticators":1} []
[2022-01-07 13:21:06] security.DEBUG: Checking support on guard authenticator. {"firewall_key":"secured_area","authenticator":"App\Security\TokenAuthenticator"} []
[2022-01-07 13:21:06] security.DEBUG: Guard authenticator does not support the request. {"firewall_key":"secured_area","authenticator":"App\Security\TokenAuthenticator"} []
[2022-01-07 13:21:07] php.CRITICAL: Fatal Error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 143360 bytes) {"exception":"[object] (Symfony\Component\ErrorHandler\Error\OutOfMemoryError(code: 0): Error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 143360 bytes) at /app/code/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php:117)"} []

EDIT: Is it possible that I need to increase the memory limit of one of the services, instead of Kimai?

If this is resource related, maybe check and increase the memory limit for the app and/or mysql service via the Cloudron dashboard.