I spoke with Max and Clayton from Passbolt about this issue, and they were really awesome with troubleshooting the issue.
Looks like I do some really freaky stuff with my implementation of Passbolt, ranging from using Apache as a ProxyPass as well as throwing MySQL somewhere else on the network.
It seems that Passbolt is engineered to run on a single instance and one of the design considerations was to force SSL usage when connecting to the server. This is a great addition and should be used in most deployments, as most people will run this on a single docker/VM instance.
However, with my freaky setup I have had to go into my passbolt configuration and disable forcing SSL by commenting out the following lines.
],
//'ssl' => [
// 'force' => true,
//]
],
];
After doing that edit, everything was all good! Thanks to the Passbolt team for helping identify this one, and I look forward to the 4.2 update!