Do not ask me why but after my holiday end of July the problem has gone:
Sun 2017-07-16 09:41:41 CEST 288 0 0 6 missing /usr/sbin/plymouthd
Mon 2017-07-17 00:01:16 CEST 2334 1000 100 6 none /usr/bin/plasmashell
Mon 2017-07-17 07:29:52 CEST 272 0 0 6 missing /usr/sbin/plymouthd
Thu 2017-08-03 20:48:24 CEST 253 0 0 6 missing /usr/sbin/plymouthd
Thu 2017-08-03 22:18:46 CEST 7165 0 0 11 none /bin/bash
Fri 2017-08-04 09:21:58 CEST 274 0 0 6 missing /usr/sbin/plymouthd
Fri 2017-08-04 17:33:12 CEST 18781 1000 100 11 none /usr/bin/dolphin
I am running now a week and see the bootscreen again.
plymouth is still 0.9.2+git20170424.6fd5c6b-1.1 so it seems that something else is updated so that I am no longer triggering this problem.