distributed: dask-scheduler does not launch Bokeh

I am seeing this starting with distributed 1.21.8 and bokeh 1.0.1 but also with the very latest versions. Starting dask-scheduler will not start bokeh any more. We are using our own compiled Python 3.6.7 and install all packages via pip. I tested it with the latest Anaconda Python 3.7 and that works fine so it seems to be an issue with our Python install …

petersen@koshuf0:~$ dask-scheduler --port 12121 --bokeh-port 12122 --host $SLURMD_NODENAME --pid-file dask-scheduler.pid
distributed.scheduler - INFO - -----------------------------------------------
distributed.scheduler - INFO - Clear task state
distributed.scheduler - INFO -   Scheduler at:  tcp://172.30.1.100:12121
distributed.scheduler - INFO - Local Directory: /loc/scratch/27074823/scheduler-xu5904ps
distributed.scheduler - INFO - -----------------------------------------------
Connection to koshu-login closed by remote host.

below Scheduler at: I should be seeing

distributed.scheduler - INFO -       bokeh at:      172.30.1.100:12122

but it is not there and bokeh is not started, there are no errors in the log. I would be grateful for any hint . There does not seem to be a --verbose or --debug option for dask-scheduler ?