UART lost in esp_bt_controller_enable()

[kbreining]

Hello,
I was just recently moving from esp-idf-v4.1-beta1 to esp-idf-v4.1. Test application was examples\bluetooth\bluedroid\classic_bt\a2dp_sink. With the beta-version of esp-idf, everything was working fine (besides the fact that, when changing i2s to internal DACs I didn't get an analog signal, which was the reason to give the new version a try). With the new esp-idf, I am "losing" the UART: after some time, putty or the eclipse-terminal just go to inactive.
With debugging via "printf", I found out, that this happens in esp_bt_controller_enable and within this function specifically in "register_chipv7_phy()". Unfortunatly this function is in a library without source code - so now I am stuck. Did anyone have the same issue and found out the reason?
Regards
Klaus