I'm sharing my story here with the hope that if this happens to anyone else, they'll know what to do. Although, this should really apply to anyone with an AVX mount.
A little over 6 months ago, I upgraded the mount for my Celestron AstroMaster 130 from the CG-3 with RA motor drive to an Advanced VX mount. The Declination axis had always been a bit stiff for my liking, even after adjusting the tension on the axis. But until just a few days ago, it had never seized up on me. I did get a new scope for Christmas though, an Explore Scientific N208CF. I think the added weight of the scope plus the 2nd 11lb counterweight is what led to the issue manifesting itself so late. The AstroMaster 130 only weighed 12.2lbs, whereas the N208CF weighs 18.4lbs. Still well under the 35lb max payload capacity of the mount.
Luckily for me, the Dec axis seized up while I was balancing the scope, instead of putting that kind of load on the Dec motor. As it happened, I remembered back to a video I had seen earlier, even before I had upgraded to the AVX mount, where another AVX owner had this same issue with two different AVX mounts straight out of the box.
The issue that causes the Dec axis of the Celestron Advanced VX mount to bind up and seize is due to the counterweight arm lock nut not having enough spacing between itself and the axis housing. The quick and easy solution is to simply invert the lock nut. The long-term solution is add a washer for spacing.
Knowing this, I was able to get my rig running smoothly again, and was still able to perform my imaging session that night. The long-term solution though, is to insert a washer into the recess of the lock nut to provide just enough spacing to prevent it from binding up and seizing again. A rubber garden hose washer will do the job perfectly. With the hose washer in place, the Dec axis glides super smooth now.
Please note, if the Dec axis for your mount is still stiff after adding the washer, the tension on the Dec axis may also need to be adjusted.