RPI-3B+: serial console shows garbage / no longer works

As you can see in the screenshots, the console basically works. Initially with the fresh image (Latest raspi_3_bullseye from git with kernel 5.10.13-1) it worked fine. I then installed "tigervnc-standalone-server xfce4 xfce4-goodies" etc via SSH. At some point I noticed that the console no longer works. Garbage appears and the console does not react any more. A login is no longer possible.

Via SSH over WLAN (5GHz) I can still get into the system. I tried to blacklist the vc4 module as described in the wiki for the Pi4. This has led to the fact that like console from the login less garbage output, but also does not work.

Is there anything I could try? Is this a known problem?