Whether you forgot the password, follow this steps to reset the app:
Note: when resetting the app you’ll find the item list empty. Please note that there are thousands of hidden items in macOS, so in order to recover your hidden folders or files, you’ll need to know where they are located, please see Recover a not listed and hidden folder solution.
1. Quit the app if it's running and be sure to be in the Finder (there must be written "Finder" next to the apple icon in the menu bar).
2. Open the Library folder.
In OS X El Capitan 10.11 or earlier:
Pull down the “Go” menu and hold down the alt/option key. Choose “Library” from the drop down list.
In macOS Sierra 10.12:
Pull down the “Go” menu and hold down the Shift key. Choose “Library” from the drop down list.
In macOS High Sierra 10.13 or later:
Pull down the “Go” menu and choose “Library” from the drop down list.
3. When you are inside the folder Library, delete this file if it is present:
Containers/com.apimac.SecretFolder/Data/Library/Application Support/com.apimac.Secret_Folder/Secret_Folder.storedata
4. Launch the app and wait until it recreates the necessary resources.
5. You're done.
The hidden files and folders are not affected in no way by this procedure. In order to recover your hidden folders or files, please see the Note below.
Related Solutions:
How to import listed items from Secret Folder 8 to Secret Folder 9
Recover a not listed and hidden folder with Secret Folder