Touch display stop working if serial connection is established

What is the problem?

My Waveshare touch display stop working if I establishe a serial connection to my 3D printer with Octoprint.

What did you already try to solve it?

Searched the WWW for a solution.
Try different USB ports on the RPi4.

Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware, what kind of hardware precisely, ...)

The touch is connected via HDMI, touch function id connected via USB to the RPi4.
If I disconnect my 3D printer from Octoprint the touch is reacting again immediately.

Octoprint 1.5.2
Python 3.7.3
Raspbian GNU/Linux 10 (buster)
Type: Pi 4B, Revision: 02, Memory: 4096MB, Maker: Sony
Anet A6 with Marlin SkyNet3D-V2.3.2
Waveshare 10,1 1024x600 Display