If you get an error message like this:

You have already activated some-gem 1.2.3, but your Gemfile requires some-gem 3.2.1. Since some-gem is a default gem, you can either remove your dependency on it or try updating to a newer version of bundler that supports some-gem as a default gem.

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You can solve this by setting PassengerPreloadBundler / passenger_preload_bundler to on.

We do not enable this by default due to this statement from the developers:

It won't be on by default because it can ironically cause a conflict with two bundlers being loaded (an old one and a new one, in that order, doesn't work well).

https://github.com/phusion/passenger/issues/2409#issuecomment-1029361123 Show archive.org snapshot