FANUC 0i-TB on KIA SKT21LMS- 410 Servo Alarm B Axis Excess Error

Gents, I have a tough one for you (or maybe not, I might get lucky).

I'm getting a 410 Servo Alarm (B Axis Excess Error) on my KIA lathe with a FANUC 0i-TB.
I picked it up used about a year ago knowing it had some issues and it's only in the past month or so I've been able to get it into a shop with 3ph to power it. The subspindle axis driver and the B motor were out of the machine when I got it, so they had obviously run into this before.

Here's what happens:
Power up lathe.
Press Standby to enable servos and zero-return all axes.
Hydraulic pump comes on, servos get powered on, all is well.
B axis (subspindle linear axis) attempts to home.
B axis jerks + or - an inch or so and throws the 410 alarm.
No alarms on any of the servo amps themselves.

If I disable B by setting it's axis number to -128, I can jog the other axes (X/Z/C) but it will NOT run normally (can't run a program, turn on spindle, etc) presumably because it wants B at home for safety.

What I have observed:

B axis servo amp had missing solder on the main power bus bars inside. I repaired this, but it may indicate a problem with the driver. The subspindle servo does move though, so if it is damages it's (a) not throwing an alarm and (b) still moving the servo a fair bit.

The encoder reads position just fine, and after doing the P-CAN procedure and resetting zero with Param #1815 the home location and soft overtravels are in (at least the ballpark of, +/- 1/4") the right place.

The motor appears to move just fine and the tech I had out (who was also stumped by this) checked the motor and agreed it seemed ok. Resistance on all phases was the same, no ground issues.

I checked all of the parameters (yes, ALL of them. Took forever) and aside from a couple of minor discrepancies (backlash, machine on time, stuff like that) it all matched up.

Here's a video of the problem too, as well as a few minutes of me rambling about the aforementioned stuff:

https://www.youtube.com/watch?v=WCqKJaD2GZ4

So my question is, what do you folks think is the problem? Should I throw a B axis drive at it and hope that's all it is? Is it a software issue I can fix by changing parameters or resetting positions of something? Know how to fully disable the B axis so I can go about my regular turning and come back to this later? Have I stumped you as well?

Let me know, and thanks in advance. I'm happy to provide more info if I missed anything.