Objects have non reset transforms

Just make sure the pivot is at 0,0,0 on all axis with the mesh at the centre and reset transforms - this sets the scale values and rotation values to 100% and 0 respectively on all axis. In 3dsMax you do this with the Reset Xform Utility. In Maya it's done with Freeze Transformations under Modify menu.

Could be an error with their system ( very unlikely) or there's indeed a problem with that particular object -"Base" .

Maybe it is indeed an issue with their system, idk . I have seen this topic come up a few times here , that's why I responded based on my personal experience with wildcat.
Some time it might really just be an issue from the software - https://www.cgtrader.com/forum/technical-q-a/non-reset-transforms

This method solved the problem. Exported a model from Maya and successfully uploaded it for review.

Make sure the model is positioned with y up, and makes sure all your scales are set to 100.
So scale your model down to 0.01, freeze transforms, then scale your model up to a 100. In other words, you submit your model with x y and z all being 100. (Make sure you export it in meters as your units. When exporting from Maya, there is a setting that you can use somewhere to manually export in meters)