The latest version (1.8.1) moves (or tries to move) the maps into the app's internal memory. This can fail if the app has lost its permission to read external storage (SD card). The maps need to be moved to internal memory because Google Play rules will no longer allow apps to read or write external memory unless they are file manager apps.
In your case, moving the maps to internal memory failed. You can move the maps one by one if you open them with your file manager. It should either ask you for an app to use to open them, or open them with Custom Maps. If your favorite file manager does not open the map files with Custom Maps, try using Google Files. It understands the kmz extension and correctly passes them to Custom Maps.
If you have a lot of maps, you can try uninstalling Custom Maps, then reinstalling it from the Play Store. Then, before you open Custom Maps, open the Custom Maps app info (long press the Custom Maps icon, and select "App info") and give Custom Maps permission to access all files ("Allow management of all files"). When you launch Custom Maps after that, it should be able to copy your maps to its internal storage, and the maps will show up in the list. Note that Custom Maps has never looked at any other area of the SD card except the CustomMaps folder, so it does not abuse this permission.
Once all the maps show up in the map list inside Custom Maps, you can delete the old map folder and the maps in it on the SD card. You can even remove the file access permission from Custom Maps. It will not need it ever again.