Specifically for the wco  stack, gem 'iron_warbler' had to be included in that particular instance's (micros_*) Gemfile. Which is a bit disappointing since I wanted libraries to be decoupled - but hey, I'm not going to fix this right now. But please be aware: it may seem that there is a separation between libraries and some libraries (namely models and content) can be installed without the rest of the stack - but that's not the case. The whole stack must be installed.

The good news is that this is a low-level internal problem, possibly caused by obsolete version of ruby being used - therefore, it may be possible to quickly fix it in the near future, and have truly separate library components in the wco stack.

.

.^.