Debian dropped support for RTSP in their VLC builds, so this is expected.
The same problem occurs with a local build. I compiled VLC version 3.0.16 Vetinari (3.0.13-8-g41878ff4f2) from source on Debian 11, after "./configure
--enable-realrtsp".
The error is in "vlc -vv 'rtsp://me:passwd@192.168.1.10:4556/cam/realmonitor?channel=1&subtype=1' " on a viable rtsp stream that is viewable in the VLC Android app and other viewers such as ffplay.
Output was saved in debug.log, but the gui error was "Your input can't be opened: VLC is unable to open the MRL 'rtsp://me:passwd@192.168.1.10:4556/cam/realmonitor?channel=1&subtype=1'. Check the log for details.", so I did:
$ grep -i rtsp debug.log
[000055d9434a65b0] main libvlc debug: configured with ./configure '--enable-realrtsp' '--prefix=/home/me'
[000055d943523210] main playlist debug: resyncing on rtsp://192.168.1.10:4556/cam/realmonitor
[000055d943523210] main playlist debug: rtsp://192.168.1.10:4556/cam/realmonitor is at 0
[00007faec0000c80] main input debug: Creating an input for 'rtsp://192.168.1.10:4556/cam/realmonitor'
[00007faec0000c80] main input debug: `rtsp://me:passwd@192.168.1.10:4556/cam/realmonitor?channel=1&subtype=1' gives access `rtsp' demux `any' path `me:passwd@192.168.1.10:4556/cam/realmonitor?channel=1&subtype=1'
[00007faeb8000fe0] main input source debug: creating demux: access='rtsp' demux='any' location='me:passwd@192.168.1.10:4556/cam/realmonitor?channel=1&subtype=1' file='(null)'
[00007faeb80011a0] main demux debug: looking for access_demux module matching "rtsp": 15 candidates
[00007faeb8001650] main stream debug: creating access: rtsp://me:passwd@192.168.1.10:4556/cam/realmonitor?channel=1&subtype=1
[00007faeb8001650] main stream debug: looking for access module matching "rtsp": 23 candidates
[00007faeb8001650] satip stream debug: try to open 'rtsp://me:passwd@192.168.1.10:4556/cam/realmonitor?channel=1&subtype=1'
[00007faeb8001650] satip stream error: Failed to setup RTSP session
[00007faeb8001650] access_realrtsp stream warning: Cseq mismatch, got 1, assumed 0
[00007faeb8001650] access_realrtsp stream debug: rtsp connected
[00007faeb8001650] access_realrtsp stream warning: only real/helix rtsp servers supported for now
[000055d943523210] main playlist debug: deleting item `rtsp://192.168.1.10:4556/cam/realmonitor'
Some package is missing somewhere, as the same code on Debian 10 works great. Any ideas, please?