no working data sync for android client

[dirk-t]

I use ATAK v5.1.0.7 (playstore) with TAK Server 5.1-RELEASE-8-HEAD.
It is not possible to get or create data feeds.
WebTAK and also winTAK v5.0.0.179 working without issues.
Server show me green connection to ATAK client in Client Dashboard.
Communication is over certificate.
Connection is green, but data sync doesn't show me anything or let me add something.
Can someone confirm this issue or has the knowlegde to fix?
I don't know, how I should debug this issue.