Why does Ae CC 2018 crash with more than 20 seconds of AVCHD/XAVC-S footage?

Unable to solve is not necessarily an error. You should only be using camera tracking on the actual frames that will be used to cover the time the inserted 3D layer will appear in the shot. For example, I recently tracked an actor walking down the sidewalk and had to replace a bunch of things in the scene. Because the actor was moving not one of the replacement elements was in the shot for more than a few seconds. I broke the long shot of the actor up into several shots that were less than seven seconds and camera tracked each one, then the shot was cut back together to create the single long shot with the completed composites. I did this because it would be nearly impossible to get a good camera solution using the entire shot, each part of the shot had different problems that had to be dealt with like reflections in windows, and camera tracking was not necessary for every single composite, but it was the most efficient for a half dozen of them.

I don't know what you shot looks like but it could be that there is just too much going on and not enough fixed geometry with sufficient parallax changes to get a good track. You can't get a camera track of a sidewalk full of people, and it can be extremely difficult to get a camera track just following a single person down a path with a camera. Camera tracking puts incredible demands on system resources. Anything you can do to lessen the burden, like trimming the shots, will help.

You may have run into a bug but, if your AE version is up to date, you more likely are just trying to solve a camera move that is too long, or in the second example, you just have a shot that will not track with your current settings. It may not track at all.