Well, after some email conversation it became clear that they thought this problem could simply be solved by installing new firmware. I explained them this is not the case. They seem to have obtained more pocketbooks without the apps, so they are going to contact Pocketbook to find out how this could have happened.
I myself finally discovered the apps on the pocketbook: they were in a trashes folder on the system! In the trashes folder I found a folder called "501" , in which I finally found the apps. Very strange!