Cinema 4D to Unity Character Animation Issues

I’ve been struggling with this for a while and can’t find the answer anywhere.
Here’s a simple character / animation in C4D built using the character builder, animated using FK keyframing:

I bring it into Unity, and this happens:

I can’t make any sense of it. The exact same issue has happened on multiple separately built characters. I can’t find a work-around.

Any help is greatly appreciated, thanks for your time.

Im kinda new to this too but I see 2 possibilities
1 your rigs parent-child relationships are not correct
2 your rotations on your joints are backwards on the characters left (when you mirror I think it makes everything the reverse)

Insure that your parent child relationships are correct
Zero out your joints
Make the mesh a child of each joint with a model like this