App shows no files and hangs before backup

I’ve just installed Memory Zone on my new HTC One S (Vodafone, not rooted or otherwise hacked). I’ve set my existing Dropbox account as my online storage. I now have a few problems, which I assume are all symptoms of the same issue:

  1. The “Internal Memory” shows very few files and no folders at all.

  2. Dropbox shows no files or folders at all, even though I have more than 10K files and many folders.

  3. When I try to run a backup, the app hangs on the “WHAT TO BACKUP?” window, showing that rotating circle forever.

I’ve tried to restart my phone; remove and install the app again; 3G and Wifi. No good, same problems.

Any idea how can I solve this issue, and make the app work?

Edit:

I’ve changed the settings so that backup can be done on both wifi and cellular connection. Made no difference. Then I’ve added a skydrive account, and the backup actually worked. So the issue seems to be related to dropbox. I have no problem using the dropbox app on my phone, and Memory Zone doesn’t complain about any problems logging into dropbox.