Solved: I read that long pressing (10s) resets the Boost-Controller. Then I pressed the button even longer while the LEGO@BOOST app on my tablet tries to connect. That brings you to some firmware update mode.(in my case from version 1.0.00.0000 to 2.0.00.0017). After this update, both Android tablets did no longer reconnect after first connection and everything works fine.
What I did not understand: Even during the firmware update the connection was sometimes lost. It took many tries until the update was done. I tried many many different positions and distances of controller and tablet. Don
t know, what I did to let it finally work.
Maybe also of interest: We have the Standard BOOST (17101) and the StarWars BOOST (75253) edition. The controller looks exactly the same, but problems occured only in the StarWars edition. Nevertheless, the StarWars edition is much more fun!