Campaign sometimes not visible on Android


I am using Xibo in the cloud (licensed) and have locally a Android Player (also licensed).
I am now testing the system, and made a campaign which consists of 5 layouts.
Sometimes the background of the layout shows, but the content is not visible.
Sometimes it is all ok.

Example 1
Example 2 - no text
Example 2a - OK
Example 3 no text
Example 3 - OK
Can you help me to look for a solution?

EDIT1: It looks like the scheduled Campaign is working one time, and then the empty screens appear.

Regards, Peter