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soundStimuli demo does not work on Linux

The soundStimuli demo does not work on my laptop (Dell latitude 5400) using Ubuntu 20.04.1 LTS. Any hint? Thanks in advance.

The output error is:

##Running: /home/martin/.local/lib/python3.8/site-packages/psychopy/demos/coder/stimuli/soundStimuli.py ##
Using PTB (with [‘portaudio’]) for sounds
PTB-INFO: Trying to suspend potentially running PulseAudio server… … status 0
PTB-INFO: Using modified PortAudio V19.6.0-devel, revision unknown
PTB-INFO: New audio device 6 with handle 0 opened as PortAudio stream:
PTB-INFO: For 2 channels Playback: Audio subsystem is ALSA, Audio device name is default
PTB-INFO: Real samplerate 44100.000000 Hz. Input latency 0.000000 msecs, Output latency 21.337868 msecs.
PTB-INFO: Will use the following logical slave-channel -> master-channel mappings for sound output to audio stream 1:
1 --> 1 : 2 --> 2 :

PTB-INFO: New virtual audio slave device with handle 1 opened and attached to parent device handle 0 [master 0].
PTB-INFO: For 2 channels playback.
PTB-INFO: Real samplerate 44100.000000 Hz. Input latency 0.000000 msecs, Output latency 21.337868 msecs.
PTB-INFO: Will use the following logical slave-channel -> master-channel mappings for sound output to audio stream 2:
1 --> 1 : 2 --> 2 :

PTB-INFO: New virtual audio slave device with handle 2 opened and attached to parent device handle 0 [master 0].
PTB-INFO: For 2 channels playback.
PTB-INFO: Real samplerate 44100.000000 Hz. Input latency 0.000000 msecs, Output latency 21.337868 msecs.
PTB-INFO: Will use the following logical slave-channel -> master-channel mappings for sound output to audio stream 3:
1 --> 1 : 2 --> 2 :

PTB-INFO: New virtual audio slave device with handle 3 opened and attached to parent device handle 0 [master 0].
PTB-INFO: For 2 channels playback.
PTB-INFO: Real samplerate 44100.000000 Hz. Input latency 0.000000 msecs, Output latency 21.337868 msecs.
done
0.2611 INFO Loaded psychtoolbox audio version 3.0.16.546732633
0.2613 INFO sound is using audioLib: PTB
0.4769 EXP Sound set volume 0.800
0.4775 EXP Sound started
1.2783 EXP Sound started
1.6790 EXP Sound started
PTB-INFO: Closing handle 3.
PTB-INFO: Closing handle 2.
PTB-INFO: Closing handle 1.
Cannot create RT messagebuffer thread: Operation not permitted (1)
Retrying messagebuffer thread without RT scheduling
Messagebuffer not realtime; consider enabling RT scheduling for user
no message buffer overruns
Cannot create RT messagebuffer thread: Operation not permitted (1)
Retrying messagebuffer thread without RT scheduling
Messagebuffer not realtime; consider enabling RT scheduling for user
no message buffer overruns
Cannot create RT messagebuffer thread: Operation not permitted (1)
Retrying messagebuffer thread without RT scheduling
Messagebuffer not realtime; consider enabling RT scheduling for user
no message buffer overruns
unknown option character l
free(): invalid pointer
#####Experiment ended. #####

Did you ever solve this? I am getting similar errors in a different situation.