When I import my character rig into a new scene, I get the following dialog box:

This box pops up eight times in a row. If I say "Yes" and try to find the missing file, it never mentions what the file type is, or where the missing file is supposed to be located...
I've noticed two details about this rig that are possibly related to error.
1)

In my imported character's layers panel, it has eight red Xs over some of the layers - specifically over the shading layer on my character's head, plus every reference layer made from that original shading layer and the folders those are contained in.
2)

When I look at this character in my content library, it has an empty "Images" folder. This is the only character I've ever made where it adds an "Images" folder once I add it to the content library. You can see in this screenshot here that I tried adding a random brush image that I renamed to "_1" in an attempt to make something happen, but this didn't change anything.
I have no idea what "_1" is, or what it wants. Does it want money? Power? I hope somebody can help, because even if I check "Don't show this error anymore" it actually still shows the error every time I open any file containing the imported character.