Had the same issue, it stemed from copying the animator used in the motion matching package, as in the Update_motionMatching function inside the animator, you will see it calls something called the evaluate chooser function, taking a reference to the original animator class, which for some reason doesnt come up as an error when duplicating. This function takes a chooser table, which is where the problem originates. The chooser table is configured to use the original animator, so you will need to remake the original chooser table, and the spare and dense versions (you cant just copy, ive found it will retain the original character reference even when changed, for some reason) do this for your animator, then back in the Update_motionMatching function, selecte the main chooser, (the original one should be called CHT_PoseSearchDataBases. Then it should be working fine.
These next ones are nested, so you will have to go in edit and fix all of those as well
There is the Spase one as well, but these are the things you need to edit, and its the same for the spase one.
If this is fine for you, then you probably have a different issue then me. Hope this helps!