I'm new to Blender and am having an issue with some joints wrinkling when moving the armature. In the attached image the left knee is exhibiting this problem, while the right knee is bending as I want it to. Weight painting doesn't seem to be helping (either heavily painted or lightly painted I still get the wrinkle).
All my other joints work just fine. Any ideas? I'm using Blender 2.80.
Picture of problem: https://imgur.com/a/7Q5kDdn
Blender file: https://tinyurl.com/wp6n6z8