P.S.: Since the interface has changed a little bit, and lately, there were some posts about this, I would like to share this clip. I recorded this evening with a new downloaded character from Mixamo and S22 as the main application, of course. I saved it and re-open the file, as that was the question here.
Clip
https://www.amazon.com/clouddrive/share/XrzIucjJThxAwg3uGvkcG4GaacYm3VMoGs6gMTWNZjM
Here is a shortlist of steps
• De-activate any animation
• Reset to Binding Pose (Weight Tag)
• BUILD Set up Character, Mixamom Control Rig.
• ADJUST will snap the control points to the Mixamo rig.
• Position the Knee joints, perhaps pull the foot back a little bit after that.
• Ignore BINDING for now
• ANIMATE will now switch to a new entry interface, Retarget Master and Weight Transfer.
• Retarget All (typically)
• Drag the Objects that have a Weight Tag in the Weight Tag List. , press Transfer Weights.
• Set the Mixamo joints to invisible
• Enable the animation back if wanted.
• Save a new copy.
Cheers