Strange model import/configuration issue

I am halfway through a tutorial and at one point I was to delete a previous (Blender) model and replace it with a new version, so as to keep character controller/animation settings. The character model was deformed so I figured I had made changes that confused it, so I wiped the model entirely and reimported the new one, but now I get animation import warnings and I can't access the configuration panel after setting the rig to Humanoid (I click the button and it selects the model but the window never opens).

Some weird things:
1 - If I import the model into a fresh project I have no problems with it
2 - If I import into this project an entirely different model that worked perfectly in another project it gets the same bug/problem.
3 - If I make a new project and import all of the assets from the old project I get the same issue.

The models in the misbehaving project have this extra "Imported Object" panel:
https://i.imgur.com/a1Ypv4C.jpg

Whereas the same model brought into a fresh project looks like this:
https://i.imgur.com/96trC7t.jpg

I should note that this model is not the one that originally bugged out so it's got to be something about that project.

My best guess is there is some temp or meta file in the assets folder relating to model import that is bugged or corrupted. Does anyone have any idea what file or folder I should clear to reset everything for model import? I could just be missing something obvious but I can't work it out.

I'm several hours into the tutorial and I really don't want to have to redo it all to get back to where I was.

Thanks.

EDIT: For anyone who might stumble on this later, I didn't solve my problem, the old project seems to be screwed. I made a new one and copied over only the most important assets and it works fine.