First:
Check your file extension, Clip Studio paint and IS are very different and incopatible, IS : tos CLIP: sut
Second - you do not have to use import button in IS you can simply drag and drop your materials. Try that.
Whatever the issue is, I highly doubt it has anything to do with IS, (imo your PC got some update or just needed a break lol) is case you do decide to reinstal it, you can simply back up your whole interface, brushes, textures and materials with "environmental backup" FILE -> Backup Environment , and import back the whole thing afterwards.
Keep in mind re installing IS will overwrite your translated files, you may want to back those up aswell, (or you'll have to run Majikuwando on the new ones)