I updated to 7.52.7 yesterday in the evening and tested the recording with the scheduler and it worked again! I'm not sure if that alone solved the problem or if it was something else. But what I mentioned is that on recordings, that didn't stopped, there was also no teletext in the video file (I'm recording as transport stream *.ts file). Not sure if there is correlation between that.

I will keep watching that the next days and weeks. But anyway, thanks for helping!