rtsp stream playing does not work in Ubuntu 21.04 and Debian 11

I am unable to play any RTSP-Stream from any XUbuntu 21.04 /Ubuntu 21.04 /Debian11 Live-Stick or my XUbuntu 21.04 which is running on my Laptop and Desktop.

I have several rtsp-sources:

    - an RTSP-Stream on my local RaspBerryPi (authentication is not needed)
    - a stream from the internet (authentication is not needed)

Both streams can be played with VLC/mplayer/ffplay on:

    - my other RaspBerryPi (RaspBerryPi OS)
    - my Windows-Laptop
    - the Android Tablets, my mobil-phone
    - XUbuntu 20.04 Laptop and Desktop-PC

Both streams can not be played with VLC/mplayer/ffplay on:

    - my XUbuntu 21.04 Laptop, XUbuntu 21.04 Desktop
    - with Debian 11 live system
    - Ubuntu 21.04, XUbuntu 21.04 Live-Version (which I have tried to have a clean system)

The problem must be the new base of Ubuntu, the new Debian 11, because there it is not possible too, to open the stream.

Two Examples, if I use the local RaspBerryPi as rtsp-Source and try to play it in VLC/mplayer under XUbuntu 21.04:

1) Error-Message from mplayer:

//-------------------------------------------------------------------------------------------------------
:~$ mplayer rtsp://192.168.178.2:8554/v0
MPlayer 1.4 (Debian), built with gcc-10 (C) 2000-2019 MPlayer Team
do_connect: could not connect to socket
connect: No such file or directory
Failed to open LIRC support. You will not be able to use your remote control.

Playing rtsp://192.168.178.2:8554/v0.
Resolving 192.168.178.2 for AF_INET6...

Couldn't resolve name for AF_INET6: 192.168.178.2
Connecting to server 192.168.178.2[192.168.178.2]: 8554...

rtsp: warning: setting NEW session: 48863bf289524adc;timeout=60;timeout=60
Timeout! No data from host 0.0.0.0
rtsp_session: unsupported RTSP server. Server type is 'VLC/3.0.11'.
libavformat version 58.45.100 (external)
libavformat file format detected.
[rtsp @ 0x7f5acb1ba180]method SETUP failed: 459 Client error
LAVF_header: av_open_input_stream() failed

Exiting... (End of file)
//-------------------------------------------------------------------------------------------------------

2) Error-Message from VLC:
//-------------------------------------------------------------------------------------------------------
...
lua art finder debug: Trying Lua scripts in /usr/share/vlc/lua/meta/art
main art finder debug: no art finder modules matched
satip stream error: Failed to setup RTSP session
main stream debug: net: connecting to 192.168.178.2 port 8554
main stream debug: connection succeeded (socket = 11)
access_realrtsp stream warning: Cseq mismatch, got 1, assumed 0
access_realrtsp stream debug: rtsp connected
access_realrtsp stream warning: only real/helix rtsp servers supported for now
main stream debug: no access modules matched
main input error: Ihre Eingabe konnte nicht geöffnet werden
main input error: VLC kann die Medienadresse 'rtsp://192.168.178.2:8554' nicht öffnen. Für Details bitte im Fehlerprotokoll nachsehen.
main playlist debug: dead input
main playlist debug: changing item without a request (current 0/1)
main playlist debug: nothing to play
dbus interface debug: Getting All properties
dbus interface debug: Getting All properties
...
//-------------------------------------------------------------------------------------------------------

3) Error-Message from cvlc:
//-------------------------------------------------------------------------------------------------------
:~$ cvlc rtsp://192.168.178.2:8554/v0
VLC media player 3.0.12 Vetinari (revision 3.0.12-1-0-gd147bb5e7e)
[000055a87a882220] dummy interface: using the dummy interface module...
[00007ff5fc001600] satip stream error: Failed to setup RTSP session
[00007ff608000c80] main input error: Ihre Eingabe konnte nicht geöffnet werden
[00007ff608000c80] main input error: VLC kann die Medienadresse 'rtsp://192.168.178.2:8554/v0' nicht öffnen. Für Details bitte im Fehlerprotokoll nachsehen.
//-------------------------------------------------------------------------------------------------------

If I try it with XUbuntu 20.04 (LIve-Linux-System via USB-Stick or on my Xubuntu 20.04 what I used before) I get this:
But it works very well!
4) Message from cvlc:
//-------------------------------------------------------------------------------------------------------
xubuntu@xubuntu:~$ cvlc rtsp://192.168.178.2:8554/v0
VLC media player 3.0.9.2 Vetinari (revision 3.0.9.2-0-gd4c1aefe4d)
[0000560c259d7250] dummy interface: using the dummy interface module...
Created new TCP socket 11 for connection
[00007f7c3c007340] main decoder error: buffer deadlock prevented
libva info: VA-API version 1.7.0
libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/iHD_drv_video.so
libva info: Found init function __vaDriverInit_1_7
libva error: /usr/lib/x86_64-linux-gnu/dri/iHD_drv_video.so init failed
libva info: va_openDriver() returns 1
libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/i965_drv_video.so
libva info: Found init function __vaDriverInit_1_6
libva info: va_openDriver() returns 0
[00007f7c34083910] vaapi generic error: profile(7) is not supported
Failed to open VDPAU backend libvdpau_va_gl.so: Kann die Shared-Object-Datei nicht öffnen: Datei oder Verzeichnis nicht gefunden
^Cxubuntu@xubuntu:~$
//-------------------------------------------------------------------------------------------------------