The most recent release of symfony/monolog-bundle allows installing monolog/monolog:~2.0, but it requires Symfony 5, while eZ Platform 2.5.x runs on Symfony 3.4.

It results in the following error when compiling container:

Symfony 5 is required for Monolog 2 support. Please downgrade Monolog to version 1.

While this is the Symfony Monolog Bundle issue (see symfony/monolog-bundle#330), we can resolve it by forcing to install Monolog version ^1.25.2 to unblock failing CIs all over the place.