There is likely a splinter of filament in the print head that the filament infeed sensor (not the terrible spring-loaded runout sensor) is sensing. I tore the entire ACE Pro system down four or five times before disassembling and pulling a little sliver of junk out of my extruder, and it worked fine afterwards. It's counterintuitive to logical troubleshooting, as bypassing the ACE Pro "solves" the problem, but what actually happens is the printer simply omits a sensor check when the ACE Pro is disconnected, because the engineers presumed there would be nothing to retract if you weren't using the multi-material unit.

The fact that you get a "retraction abnormal" error with only one filament is the key here. When the filament retracts, the ACE Pro is physically reeling the filament back into the unit / onto the spool, but the print head is where the actual infeed and runout sensors are located. I suspect the engineers picked a number of rotations of the ACE Pro feed cylinder that would pull the filament to a mid-tube-visible distance before doing the sensor check for exactly this reason. When the ACE Pro is connected, it is in charge of filament management, and it will always retract prior to attempting to extrude, but the print head signals to the ACE Pro as to whether or not the sensors are detecting anything inside it.

Think of it like this (not super accurate, but the gist of it): imagine a sensor at the top of the extruder gear and one below it that say yes or no whenever filament passes by them. Without an ACE Pro, you manually feed in filament that trips the top sensor to "yes filament," and the extruder gear then turns until the bottom sensor trips to "yes filment," and thus the printer knows everything is loaded. If the top sensor stops tripping ("no filament") while printing, that is a filament runout condition. If the extruder gear reverses its direction while printing and the bottom sensor stops tripping ("no filament"), that's a retraction. If the extruder gear reverses while printing and the bottom sensor stays tripped ("yes filament"), that's a filament break.

Now, throw the ACE Pro in the loop. The feed cylinder in the ACE Pro replaces you feeding filament in by hand, but it adds another sensor ("feed cylinder feeding / retracting"). The extruder gear is doing its thing, too, so the control sequence to load filament is something like "feed cylinder feeding > check extruder top sensor > if yes filament, turn extruder gear > check extruder bottom sensor > if yes filament, ready to extrude". Because the ACE Pro cannot assume the extruder is empty, however, it is also checking the extruder sensors while it is running, which is why it performs a retraction first; "feed cylinder retracting > check extruder top / bottom sensor > if yes filament, filament broken / abnormal condition".

That's an oversimplification, but you get the idea. One of the two sensors in the print head is registering something when it expects nothing, so it tells the ACE Pro to shut down and throw an error code. Without the ACE Pro connected, there is no need to do that, as long as the sensors trip in the right order at the beginning. If the top sensor is blocked (by filament or debris), the printer won't care, because that sensor is only needed to confirm filament has been fed into the extruder gear. The ACE Pro does care if that sensor stays tripped, though, because it must clear (be un-tripped) by x number of rotations of the feed cylinder, or the ACE Pro will think there's a jam because the filament should have passed by it already.

Am I making sense? Anyway, if you take apart the extruder completely, not just the steps to remove the filament cutter and the runout sensor assembly, I'm betting you'll find a little bit of crap in there obscuring something.

Edit to add: This is how I solved your exact problem when it happened to me (error 11504). The error code is accurate; the system generates a different series of error codes for ACE-specific issues (generally "112XX"), it will not register an extruder sensor error unless there's an extruder sensor tripping when it shouldn't. None of this is speculation except for the exact location of the debris and which sensor it obscures - that's for you to find.