First try right clicking on your contents folder and select “fix up redirectors in folder” not sure if that will work in this case but it’s worth a try.
To avoid problems like that instead of just copy pasting the files from your old project to the new one open your old project, right click the things that you want to transfer (in this case the materials, it will automatically include everything the materials used) go to asset actions and migrate them - you have to chose the content folder of your new project to avoid redirect problems once they are migrated you are free to rename and change the folders from within the engine. If you do it with copy paste you have to make sure that the folders are with the same directory names compared to the project file or you’ll have redirect problems like textures not showing up. Not sure why the folder looked empty, if it was in the content browser it probably didn’t refresh properly sometimes that happens to me and changing to a different directory and going back fixes it.
