For those of you who don’t understand German, I have translated my post into English 
I have been using a Raspi 3B and the original 7" touch display for many years. Since the new Volumio version (v 4.069) cannot be installed via an update but only via a complete reinstallation, I was hoping that the installation of both Volumio and the display plugin would now be quick and easy. And indeed, the installation of Volumio was much faster than before. The installation of the touch plugin (v 3.6.0) was also successful. However, disillusionment set in when the display did not work. Everything remained dark.
As in version 3, I was able to get the display working by editing the “userconfig.txt” file.
To do this, I removed the SD card from the Raspi and opened the userconfig.txt file on my PC with the editor. The only entry there was
Add your custom config.txt options to this file, which will be preserved during updates
For those of you I added the following to this entry (See the userconfg.txt file I have attached.)
After saving the file, the SD card was reinserted into the Raspi and restarted. Now the plugin/display is working. It may be necessary to adjust the display brightness settings via the plugin.
I don’t know what the individual lines in userconfig.txt do, and I’m not familiar with Linux. “My” solution is based on hours of searching the internet.
The decisive factor was the English-language entry at 3.779 - after upgrade Raspberry Pi touchscreen display does not work any more - #36 by nerd
userconfig.txt (331 Bytes)
I hope that my contribution has been helpful to some of you.