iMessage registration fails with "INTERNAL: Server Error", console has "Account registration failed: ... Failure reason: 18"

So I've already solved this, just documenting it for the search engines and future sufferers. I spent a while trying to get this to work and didn't find much about it.

Here's the situation I had:

• SMBIOS and other RT variables were configured correctly (serial valid but not registered, etc)
• Valid BSD interfaces (en0 and en1 correctly registered, both BCM wifi and onboard ethernet were functional with AirDrop as well)
• All cache and iMessage related folders had been nuked
• Disk Utility had been used to repair permissions
• iMessage had worked previously on this setup back in High Sierra but not longer worked in Mojave

Despite this, Messages failed to sign in with the error INTERNAL: Server Error in the GUI and the error account registration failed (<private>). Failure reason: 18 from identityservicesd.

The solution? rm -rf /Library/Preferences/SystemConfiguration.

I didn't originally try this because it was only suggested for cases where your BSD interfaces were incorrect (i.e. no en0). As it turns out, there's another layer to BSD interface configuration. en0 apparently must be your ethernet NIC and en1 can be your wireless NIC. I believe this issue came about when I installed a new wireless card (to get bluetooth and AirDrop working) back on High Sierra. Somehow the system didn't reconfigure correctly and so while I was able to stay registered in High Sierra, it could not re enroll on Mojave.

In the future, I guess I'd recommend clearing SystemConfiguration anytime you make hardware network changes because macOS doesn't always get the memo/configure itself correctly.