Here is an exact how-to of our head ios developer:
1. restart your device
2. open Excel and open an existing sheet or create a new one.
3. within the save dialog, you will find a "more" button.
4. Now tap "Locations" in the upper left corner and select Boxcryptor if it is present.
4a) tap "more" and enable Boxcryptor as location
I hope this helps a bit.