ToneLune I have some difficulties understanding the link between Bazaar and your issue with FastCGI headers... 🤔 the error you get is still the one you copied in the first post ?
Maybe this is a permissions-related issue ?
You may open an issue at Bazaar, but we'll need solid replication steps, for example on a DigitalOcean droplet, otherwise we probably won't be able to do anything about it.
Also last thing I notice, you say PHP 7.2 ? That's not supported by Flarum beta 7, so not something we support either at this time. Are you indeed running Flarum beta 7 on PHP 7.2 ?