I had responded to your query in another channel. Please don’t post the same question in two or more different channels.
Here also i will write the same respose for some future reference for the users.
This problem has occured in the mobile device which is run by Android 6.0. But the same scenario(which you are trying) is working fine with the later version of Android(>=8.0).
If you directly open the file from Dropbox using the AxCrypt unlock icon, it’s throwing the file does not exists exception.
There is a misleading between the two apps in the older version of Android. We are checking on this issue and I will provide a simple temporary solution to work for now.
Download the file in your mobile from Dropbox and then try to open the file. Now AxCrypt app can open the file without any issues.
You can download the encrypted file anywhere in your mobile and open the same with the AxCrypt app.