What is the specific problem with the base classes? In the blueprint editor at the top is a button called “Class Settings”. If you click that, the details panel will show “Parent Class” at the top (and you press “Class Defaults” to get back).
Before you go crazy trying to fix this, you should probably try a 100% fresh build (if you haven’t already). Sometimes the intermediate files get corrupted. The easiest thing to do is to go in the editor (your project) and choose File->(Package project)->(Zip up project). Make a zip and then unzip it into a new directory. Try building that one and see if anything changes.