Another update. I have gone through the whole process in the Apple website again by following this guide:
I documented everything with screenshots. Hopefully somebody can tell me if I've made an error somewhere.
1. I created a brand new private sign key in an empty folder (that I specified in Configure Paths in B4i)
2. This is my keys folder so far
3. I have no new devices to add so that step is skipped
4. Logged in to Apple website
5. I created a new App ID
There's a long list of options here, but I chose App IDs, because that best matched the instructions.
I chose App.
I gave it a description and a wildcard bundle id. The instructions say to just enter * here, but that isn't accepted by Apple, so I prefix it with b4i. I selected no capabilities/app services.
I confirmed the App ID.
6. I created a certificate
The instructions say to create a production certificate, but that phrase isn't used by Apple anymore. (It is used when talking about certificates for push notifications and watchkit services - clearly not what I'm looking for.) But the guide says to pick a certificate of type "App Store and Ad Hoc". I picked "iOS Distribution (App Store Connect and Ad Hoc)" because it matched the guide best.
I uploaded the certSigningRequest.csr that I created just a few minutes before.
I downloaded the generated certificate as default.cer into the keys directory.
This is what the keys folder looks like now.
7. I created a privisioning profile
The guide said to choose "Distribution -> Ad Hoc", and this exact option is available so I picked that.
I picked the App Id created above.
I picked the certificate created above.
I selected all devices.
I named and generated the provisioning file.
I downloaded the provisioning file into the keys folder.
This is what the keys folder looks like now.
After this I followed this guide to build B4i-Bridge.
8. Create a new B4i project for B4i-Bridge
I set the package name identical to the guide, which also matches the b4i.* app id I used above.
Not part of the guide, but I verified that I'm using the hosted builder, using the temporary username I have.
I chose to Build B4i-Bridge App, and failed with the same error message:
B4i Version: 8.51
Parsing code. (0.01s)
Building folders structure. (0.01s)
Running custom action. (0.03s)
Compiling code. (0.02s)
Building designer app. (0.02s)
Building Xcode project. (0.09s)
Preparing project for builder. (0.36s)
Project size: 0.23 MB
Sending data to remote compiler. Error
Error: security: SecKeychainItemImport: MAC verification failed during PKCS12 import (wrong password?)
I also tested with my local builder, and got the same error message.