I was watching the behavior of the WiFi LED on the board. Usually it takes not longer then 3-5 seconds until it goes from yellow (no connection) to green (connection established). So I set an initial delay of 10 seconds at the begin of my program and check the board´s IP address then. Successful, so far. I´ll try to improve my code in the next few days, cause this delay somehow feels not very smart.
However, since the startup time of the program is not critical for my project I could also live with the delay.
Daniel