Try to import the backup manually:
Rename the backupfile and add ".zip" to the filename. You can now open the backupfile with a filemanager or winzip and see the encrypted files. Move the folders
.do_not_delete_2 from the backup into the Privary data folder. It is located directly on your storage.
You may will be asked to migrate the folder - confirm. Files from the backup are now manually imported to the current vault and will be visible, if the password / PIN matches with the password used to encrypt the files.
P.S.: The .privary folder is located on your device storage or sd-card (if activated from the settings). It's in a hidden state. You can make it visible if your file manager app can show hidden files.